Absorption of Pharmacological Agents Through Different Routes of Entry

Absorption of small organic molecules is usually passive, but it may involve a facilitated process or an active process that requires energy. The factors that affect the absorption of chemicals from the gastrointestinal tract are listed in Table 5–1.

TABLE 5–1

Factors Affecting Absorption of Chemicals.

The venous blood supply of the entire gastrointestinal tract except the rectum goes directly to the liver, where absorbed compounds may be metabolized (first-pass metabolism). This observation may be explained by the high prevalence of CYP3A4 in the upper part of the intestine and its absence from the colon and rectum. Drugs given through the rectum (suppositories) and urinary bladder do not go through significant first-pass metabolism (Buyse et al., 1998). Interestingly, drugs applied to the vagina accumulate to a greater extent in the uterus than when administered by other routes; that is, they have a “first-uterine-pass effect” (Bulletti et al., 1997; Mizutam et al., 1995).

Gastrointestinal Tract Absorption of drugs from the stomach is affected by a variety of factors (Table 5–1). (For a general review of the effect of gastrointestinal motility on the absorption of drugs, see Hebbard et al. [1995].) Gastric emptying (Malagelada et al., 1993) can be measured by several techniques, of which transit times determined with a radiolabeled liquid or solid meals provide the best-validated and clinically meaningful measurements (Camilleri et al., 1998).

Differences in age, sex, body mass index, phase of menstrual cycle, and type of meal consumed lead to large inter- and intrasubject variabilities. The preponderance of evidence (Bennink et al., 1999; Datz et al., 1987; Gryback et al., 1996; Hermansson and Silvertsson, 1996; Hutson et al., 1989; Knight et al., 1997; Tucci et al., 1992) supports the conclusion that females empty solids more slowly than men; however, others have found that all transit variables are unaffected by sex (Madsen, 1992). The gastric emptying of liquids has also been reported to be slower in women than men (Datz et al., 1987; Mohiuddin et al., 1999); however, others have found no differences (Bennink et al., 1998). In addition, young people empty their stomachs faster than elderly people do (Jann et al., 1998; Moore et al., 1983; Teff et al., 1999). The small-bowel transit time of solids and liquids do not differ between the sexes (Horowitz et al., 1984; Madsen, 1992), but differences are seen between old and young subjects (Bennink et al., 1999; Madsen, 1992).

Slower gastric emptying in females does not likely affect the absorption of most solid drugs since most absorption takes place in the small intestine. The rate of absorption of enteric coated forms is delayed (Mojaverian et al., 1987). Drugs with narrow therapeutic indices are most likely to be harmful (Greiff and Rowbotham, 1994) or lack efficacy in persons with slow rates of gastric emptying. A slow rate of gastric emptying also decreases the level of absorption of alcohol.

Progesterone may be responsible for the slower rate of gastric emptying in women (Gill et al., 1985, 1987; Hutson et al., 1989; Mathias and Clench, 1998; Riezzo et al., 1998). Females are less sensitive than males to muscarinic blockade of stomach motility, possibly because of differences in autonomic tone (Teff et al., 1999). It is of interest that 80 to 90 percent of diabetic patients with gastroparesis (paralysis of the stomach) are females (Bennink et al., 1998).

Basal acid output, pH, and gastrin secretion are independent of sex (Bernardi et al., 1990; Dressman et al., 1990; Straus and Raufman, 1989) and age (Russell et al., 1993). In patients with gastroesophageal reflux, however, the mean basal level of acid output is greater in males than females (Collen et al., 1994).

Transport systems within the gastrointestinal tract may have significant effects on drug absorption. For example, a component of the intestine and liver, P glycoprotein (Pgp), is a transmembrane efflux protein that actively transports many compounds including drugs out of cells. The livers of males express twofold larger amounts of Pgp than the livers of females (Schuetz et al., 1995). This suggests that males transport drugs out of hepatocytes more rapidly than females, decreasing the time for biotransformation, although further studies are needed. The absorption of many drugs might be affected if a sex difference in intestinal Pgp activity also exists, and it is important to determine whether such a sex difference exists. Drug absorption rates from the rectum may also be sex dependent. Females absorbed one of the two specially prepared ondansetron suppositories (an antiemetic) differently than males (Jann et al., 1998), although this could be attributed to normal variability.

The menstrual cycle has no effect on motility in the esophagus (Mohiuddin et al., 1999) or on whole-gut transport (Kamm et al., 1989). However, evidence for a menstrual cycle effect on gastric emptying is conflicting (Gill et al., 1987; Horowitz et al., 1985; Mones et al., 1993; Parkman et al., 1996; Petring and Flachs, 1990).

Skin Gels, ointments, creams, or patches deliver small organic compounds through the skin (Brown and Langer, 1988; Xu and Chien, 1991). Transit through the stratum corneum (outer skin) barrier often requires the use of absorption enhancers (Kanikkannan et al., 2000).

The level of transepidermal water loss, a measure of epidermal barrier permeability following injury to and recovery of the stratum corneum, does not differ between males and females or between Caucasians and Asians (Reed et al., 1995). Dark skin recovers from an injury more quickly than lightly pigmented skin. In a clinical trial of transdermal clonidine for the treatment of hypertension, blood pressure reduction was independent of race, ethnicity, sex, and age (Dias et al., 1999). In vitro, transdermal absorption of fentanyl and sufentanil (analgesics) was neither age nor sex dependent (Roy and Flynn, 1990).

Pulmonary tract Absorption of drugs via the pulmonary tract varies with breathing rate and depth (ventilation) (Gonda, 2000). Progesterone may be a ventilatory stimulant. Females have greater minute ventilations and lower tidal volumes than males, and the ventilatory response to high carbon dioxide levels is greater in males (White et al., 1983). A drug, ethionamide, given orally to healthy and ill men and women appeared in equal concentrations in alveolar cells of both sexes (Conte et al., 2000). Women have higher ventilatory responses in the luteal phase than in the follicular phase of the menstrual cycle.

Protein Binding

Most small organic compounds bind to albumin or α₁-acid glycoprotein (AAG) and less frequently to alpha, beta, and gamma globulins, lipoproteins, or erythrocytes. AAG binds with a high affinity to basic drugs. Albumin binds to acidic drugs in a complex in which the drug readily dissociates to maintain an equilibrium between the bound and unbound (free) fractions. The unbound fraction is in equilibrium with the receptor (Anton, 1960; Shoeman and Azarnoff, 1975). Thus, the degree of binding of drugs to plasma proteins can influence their dispositions (Gillette, 1973).

The plasma protein binding of enantiomers (mirror-image compounds) in racemic mixtures (containing both enantiomers) may differ, and selective binding of the enantiomers does occur (Gross et al., 1988; Walle et al., 1983).

The level of AGG, an acute-phase reactant, increases in patients with infections, cancer, and rheumatoid arthritis. Decreased albumin levels or increased AAG levels occur in patients with renal, liver, and thyroid Crohn's disease, myocardial infarction, cancer, and burns (Reidenberg and Affrime, 1973). Increased levels of unbound drug occur in patients with uremia (Garland, 1998; Reidenberg and Affrime, 1973) and cirrhosis (Goldstein et al., 1969). It is not known whether sex differences exist in these circumstances.

In a very small study (nine women each examined through one menstrual cycle), the concentration of AGG was higher on day 4 of the menstrual cycle than on days 12, 20, and 28 (Parish and Spivey, 1991), but the study had very significant inter- and intrapatient variabilities. The level of sex hormone binding globulin has also been found to increase during the luteal phase of the menstrual cycle (Plymate et al., 1985).

The concentration of a drug in the fetus is a function of the concentration in the mother, placental permeability, fetal drug clearance, and differences in the levels of protein binding between maternal and fetal plasma (Boulos et al., 1971). The placental transfer of lipophilic drugs is good, but the transfer of hydrophilic drugs is slow.

The concentration of protein is significantly lower in the fetal circulation than in the maternal circulation during early pregnancy, but the concentration in the fetus exceeds that in the mother at term. The maternal AAG level is very low before 16 weeks of gestation and thereafter increases at a constant rate to a fetal concentration-maternal concentration ratio of 0.37 near term (Perucca and Crema, 1982). Examples of circumstances in which plasma protein variables affect drug concentrations are shown in Table 5–2.

TABLE 5–2

Differences in Drug Concentrations Between the Mother and Fetus and Between Males and Females.

Body Composition

Male-female differences in body fatness may account for the increased volumes of distribution for lipophilic drugs (such as benzodiazepines) in females (Parker, 1984; Sciore et al., 1998) and for alcohol in males (Loebstein et al., 1997; Parker, 1984; Petring and Flachs, 1990).

The level of total body water decreases with age because of a disproportionate decrease in intracellular water levels (Kashuba and Nafziger, 1998; Phipps et al., 1998). It is important to adjust body composition models not only for sex but also for age and body size (Kasuba and Nafziger 1998; Phipps et al., 1998). (It is important to note, however, that some sex differences in pharmacokinetics reported in the literature are a result of differences in weights between males and females receiving the same fixed dose of the drug. Thus, pharmacokinetic parameters should be corrected by weight before concluding that a sex difference exists. The effects of differences in body fat, as noted above, can confound weight issues and should also be considered.)

Biotransformation

Sex has a complex effect on the pharmacokinetics of drugs metabolized in the liver (Harris et al., 1995; Yonkers et al., 1992). Temazepam and oxazepam, benzodiazepines that are metabolized through conjugation, are cleared faster by males (Divoll et al., 1981; Greenblatt et al., 1980; Smith et al., 1983). Alprazolam (Kristjansson and Thorsteinsson, 1991) and diazepam (Greenblatt et al., 1980) are metabolized via an oxidative mechanism and are cleared faster by females. Nitrazepam (Jochemsen et al., 1982) is metabolized via reduction of its nitro group, and its metabolism shows no sex differences. Thus, sex affects differently even drugs within the same pharmacological class and drugs with the same structures.

Cytochromes P450 The cytochromes P450 (CYPs) are a superfamily of at least 17 isozymes that modulate the oxidative metabolism of drugs in the liver (Wrighton and Stevens, 1992). CYP1, CYP2, and CYP3 are thought to be responsible for most hepatic metabolism of drugs. The CYP3A4 subfamily is the most abundant of the CYPs in the human liver and is responsible for the metabolism of cyclosporine, quinidine, erythromycin, dapsone, and lidocaine (Harris et al., 1995; Wing et al., 1984). The major CYP isoform found in the human embryonic, fetal, and newborn liver is CYP3A7. The activity of CYP3A4 is very low before birth but increases rapidly at birth and reaches 50 percent of the level in adults between 6 and 12 months of age. This maturation of drug-metabolizing enzymes is the main factor for age-associated changes in nonrenal drug clearance (de Wildt et al., 1999).

Studies with tirilazad (an antioxidant) (Hulst et al., 1994), erythromycin (Watkins et al., 1985), and diazepam (Greenblatt et al., 1980) suggest that females have greater CYP3A4 activity, but studies with other probe drugs yield conflicting results. Drugs metabolized by CYP3A4 are extensively cleared by females, whereas drugs cleared by other isozymes are usually cleared faster by males (Harris et al., 1995). The sex-specific differences in CYP3A4 activity are related to estrogen and progesterone, which regulate the activity of CYP3A4 at the gene level (Harris et al., 1995). However, the metabolism of ranitidine, also metabolized by CYPs, shows no sex difference (Abad-Santos et al., 1996). The metabolism of some drugs eliminated through conjugation shows sex differences (Divoll et al., 1981; Greenblatt et al., 1980, 1984; Macdonald et al., 1990; Miners et al., 1983, 1984).

Excretion

The kidney is the major organ of drug excretion. Drugs diffuse in their un-ionized form across the kidney glomeruli and tubules or are secreted and reabsorbed by active tubular transport systems. Drugs are also excreted in the feces if either the drug has not been absorbed from the gastrointestinal tract or it is excreted from the liver into the bile in the intestinal tract. Reabsorption (enterohepatic circulation) may occur as the excreted drug travels through the intestines.

Males have higher levels of creatinine in serum and urine and higher rates of creatinine clearance (CL_CR) than females. The difference is related to the greater lean body mass of males (James et al., 1988). In a three-way crossover study with young and elderly men and women, the renal clearance of amantadine was significantly inhibited by quinine and quinidine only in the male subjects. There were no age-related effects (Gaudry et al., 1993). The mechanism of this interaction is probably related to the differential effects of quinine and quinidine on the tubular excretion rate of cations (Charney et al., 1992). A physician would not need to consider this interaction when treating an elderly woman with quinidine for muscle cramps who developed influenza and was prescribed amantadine; a lack of attention to this drug-drug interaction in a male, however, could lead to a significant adverse reaction.

CL_CR is lower during the first week of menses and increases by week 4 by 20 percent. Overnight CL_CR measured three times a week during 11 menstrual cycles found the median CL_CR to be 7.3 percent higher during the luteal phase than during the follicular phase. Similar changes were found when intravenous chromium 51-labeled EDTA was used, a more accurate measure of the glomerular filtration rate (Paaby et al., 1987a,b). Estradiol and estriol do not affect the glomerular filtration rate, urine flow, renal plasma flow, or tubular reabsorption in humans (Christy and Shaver, 1974; Davison, 1987). CL_CR was found to be slightly increased only in the midluteal phase in another study (Phipps et al., 1998). However, the changes were attributable to changes in creatinine excretion and are not considered clinically important. It is surprising that investigators are still attempting to determine the effect of the menstrual cycle on CL_CR.

The renal clearance of the aminoglycoside antibiotics tobramycin (Nafziger et al., 1989) and amikacin (Matsuki et al., 1999) are not significantly altered during the menstrual cycle. In a recent review, Kashuba and Nafziger (1998) reported that most published studies have been conducted with small numbers of women and limited numbers of menstrual cycle phases within one menstrual cycle. In addition, studies of the effects of estrogen or progesterone on renal clearance are limited and their results are contradictory. They further conclude, “there are no demonstrated clinically significant changes that occur in the absorption, distribution or elimination of drugs” during a normal menstrual cycle (Kashuba and Nafziger, 1998, p. 204). Beierle et al. (1999) concluded that sex-related differences in the renal clearance of drugs is generally only of minor importance. However, a meta-analysis of 10 studies with 172 healthy volunteers administered an oral therapeutic dose of the antibiotic fleroxacin provided evidence that the volume of distribution/systemic availability ratio (V/F) is 20.4 liters greater in males than females and that the clearance/systemic availability ratio is 10.8 milliliters per minute (ml/min) greater in males than females (Reigner and Welker, 1996).

Pharmacodynamics

Receptors are macromolecules on or in cells to which a drug binds to initiate its effect. Continued stimulation of a receptor may lead to its downregulation or desensitization (refractoriness). Hyperreactivity may also occur and may result from long-term administration of antagonists or the synthesis of additional receptors. Drugs may also act as substrates for enzymes or may inhibit enzymes competitively or noncompetitively.

Although there is marked interindividual variation (Levy, 1998), pharmacodynamic differences between the sexes do occur. Several examples are shown in Table 5–3. (For additional examples, see Table B-1 in Appendix B.)

TABLE 5–3

Receptor, Enzyme, and Structural Differences Between Males and Females.

Sex Differences in Adverse Events

Drugs from classes as diverse as antihistamines (terfenadine), antibiotics (erythromycin) (Makkar et al., 1993), and antiarrhythmic drugs (d,l-sotalol) (Kuhlkamp et al., 1997; Lehmann et al., 1996) can induce a potentially lethal cardiac rhythm called torsades de pointes. The risk of having this drug-induced complication is far greater in females than in males. Hypokalemia, hypomagnesemia, bradycardia, and the baseline QT¹ interval and the degree of QT prolongation (electrocardiographic variables) increase ones susceptibility to this event (Napolitano et al., 1994). As females possess a longer average electrocardiographic QT interval-corrected heart rate (QT_c) than males, a difference not found in the newborn (Stramba-Badiale et al., 1995), sex differences in cardiac ion channel function account for the increased incidence in females (Rautaharju et al., 1992). In clinical studies it was found that quinidine, a drug that induces QT_c interval prolongation, also induces a greater prolongation of the QT_c interval in females (Benton et al., 2000). Furthermore, in animal models, estrogen has been found to prolong the QT_c interval, affecting cardiac repolarization (Drici et al., 1996). Recent studies with women suggest that drug-induced QT interval prolongation may be affected by the phase of the menstrual cycle (Rodriguez et al., 2001).

Studies of isolated ventricular myocytes indicate that active drugs block the delayed rectifier potassium channel (I_k) (Wesche et al., 2000). Adjustment of dosage for body weight or surface area does not ameliorate this problem. Recently, the U.S. Food and Drug Administration (FDA) has required label warnings for drugs that prolong the QT_c interval. One drug with this effect, cisapride, was removed from the market.

Although the prolongation of the QT_c interval by the d-sotalol enantiomer is said to be independent of dose and sex (Salazar et al., 1997), the small population size of the study (four males and four females, which is not sufficient to be able to draw general conclusions) exemplifies the inadequacy of the available literature on this topic.

Genes associated with the long QT syndrome and sudden death have mutations that affect ionic currents involved in the control of ventricular rhythm. Only those subjects with a mutation affecting the sodium-channel ionic current have sufficient prolongation of the nighttime QT interval to increase arrhythmic risk (Stramba-Badiale et al., 2000). The sex difference in QT_c interval observed in adults is not seen on the 4th day of life (Stramba-Badiale et al., 1995).

Other sex-related differences in side effects have also been noted. For example, females are twice as likely as males to develop a cough while taking angiotensin-converting enzyme inhibitors (Kubota et al., 1996; Strocchi et al., 1992).

Sex Differences in Effectiveness

Sex differences in the therapeutic response to the 5HT3 antagonist alosetron have been reported. FDA has recently approved alosetron for the treatment of nonconstipated irritable bowel syndrome in females. It is ineffective in males, but the reasons for this are unknown (Bardhan et al., 2000; Camilleri et al., 2000). Another example of sex differences in the therapeutic response to a compound that acts by a serotonergic mechanism is sertraline. FDA recently approved sertraline for the treatment of posttraumatic stress disorder in women. Two multicenter, placebo-controlled trials for the treatment of posttraumatic stress disorder demonstrated its effectiveness in females but not in males (Henney, 2000).

Differences in the rates of serotonin synthesis in the brains of female and male volunteers have been measured by positron emission tomography. This technique allows a direct measurement of the rate of serotonin synthesis in the living brain. The rate of brain serotonin synthesis was found to be 52 percent greater in male subjects than female subjects. The researchers suggest that this marked difference in rates of serotonin synthesis could contribute to the higher incidence in women of major unipolar depression and other psychopharmacologies in which serotonergic mechanisms are implicated in the pathophysiology of the disease. However, a few postmortem studies found no significant sex differences in serotonin levels (Arato et al., 1991; Dean et al., 1995; Nishizawa et al., 1997).

Innate and Adaptive Immunities

This section reviews differences in innate and adaptive immunities between females and males. Gonadal hormones partly control these normal defense systems. The literature on the nonhormonal effects of sex on mechanisms of innate and adaptive immunities is sparse, however.

Adaptive immunity varies markedly by sex; innate immunity varies less. Mature young females mount more vigorous immune responses than others. Whole-organism elements of inflammation and immunity include cycling, hormones, growth and nutrition, life stages, and life events (discussed below). Of these, only hormones have been extensively studied.

Chronobiological events occur over short intervals (e.g., brain waves and heart beat), days (sleep cycle), weeks (menstrual cycle), or years (seasonal cycles) (Young, 2000). Male and female chronobiologies, that is, menstrual or estrous cycles, should thus be considered separately from the associated hormonal changes. Both sleep deprivation and jet lag, examples of chronobiological events, are immunosuppressive (Ishida et al., 1999). Hypothetically, a chronobiological effect on immunity may occur in menstruating females or may render a fertile female vulnerable at certain times of the month. Studies that have compared exogenously cycled and noncycled castrated animals have not been done.

Leukocytes and their products constitute the innate immune system. Leukocyte function, which is assessed by cellular synthesis of degradative enzymes (Kuslys et al., 1996), oxidative metabolism (Garcia-Duran et al., 1999), and adherence and phagocytosis (Ito et al., 1995; Josefsson et al., 1992; Miller, 1999; Mondal and Rai, 1999; Spitzer, 1999; Spitzer and Zhang, 1996a,b; St. Pierre Schneider et al., 1999), is modulated by estrogen. Some estrogen-induced changes increase the levels of functioning of leukocytes in females; others decrease their level of functioning. Overall changes are small, and sex differences in levels of leukocyte functioning probably do not affect human illness.

The adaptive immune response includes activation and suppression of T and B lymphocytes, macrophages, and dendritic cells; secretion of their cytokine products; production of immunoglobulin antibodies; and activation of the complement and coagulation systems.

The adaptive immune response of females, as measured by determination of the level of cell proliferation or immunoglobulin levels, is more vigorous than that of males. The sex differences are relatively small. Comparable differences are seen between Caucasians and African Americans and between young and old individuals. Persons with chronic inflammatory illnesses have activated immune systems. The implication of the differences between males and females is unknown.

As a rule, estrogenic hormones upregulate and androgenic hormones downregulate the cellular effectors of human adaptive immunity: lymphocytes, macrophages, and dendritic cells (Ahmed and Talal, 1999; Kanda et al., 1999). The adaptive immune response varies during the menstrual cycle. However, most experiments on immune cells examine specific questions (e.g., does estradiol upregulate expression of a certain substance without considering physiological age, menstrual cycle, or other variables (Lockshin, 1999). Reviews are readily available (Cannon and St. Pierre, 1997; Cutolo et al., 1995; Draca, 1995; Gaillard and Spinedi, 1998; Kammer and Tsokos, 1998; Marchetti et al., 1998; Wilder, 1995). Representative recent data are displayed in Table 5–5.

TABLE 5–5

Sex Differences in Immunocytes as Determined in Representative Recent Studies.

The hypothalamic-pituitary-adrenal-gonadal axis, which exerts important control on the adaptive immune system, differs between males and females. Exercise, stress, and depression all downregulate immune function (Irwin, 1999; Nehlsen-Cannarella et al., 1997). Since each of these differs between the sexes, sex differences in the resultant illness may occur.

Immunization

Sex-specific studies of responses to immunizations with vaccines show intriguing serological differences (differences in circulating antibody levels) but few clinical differences between females and males (Table 5–6). Sex differences in serological response are not generalizable among organisms. Adverse systemic reactions to immunization, particularly arthritis, are more common in females.

TABLE 5–6

Sex Differences in Immunization.

Pregnancy

Estradiol levels increase 100-fold and estriol levels increase 1,000-fold during human pregnancy. Within these ranges, estrogens upregulate immune functions, but clinically evident immunological changes during pregnancy are small. Pregnancy-specific proteins suppress lymphocyte function. Changes differ at different stages of pregnancy, with no apparent general pattern. Cutaneous and humoral immune responses to specific microbial antigens are selectively depressed, as are leukocyte chemotaxis and adhesion. Overall, during pregnancy the immune system deviates markedly from that during the nonpregnant state; the long-term effects, if any, of this deviation on women's biology or health are unknown. Specific infections, such as those caused by the measles virus, appear to be particularly virulent in pregnant women.

The striking sex difference in many autoimmune disorders is incompletely understood. A new aspect under study is the role of fetal cells transferred to the maternal blood (microchimerism) (Bianchi, 2000) and their persistence postpartum, in some instances for decades, in the pathogenesis of scleroderma (also called systemic sclerosis, a connective tissue disorder that leads to fibrosis of skin and internal organs). In women with scleroderma who had given birth to at least one son before disease onset, male DNA was detected more often and in larger amounts in their blood than in their normal sisters who had given birth to one or more sons (Artlett et al., 1998; Nelson, 1998; Nelson et al., 1998). Persistent microchimerism of maternal lymphocytes in the circulation of offspring occurs as well, but its relationship, if any, to autoimmune disorders, is not yet known. The implications of the association between autoimmune disorders and fetal microchimerism are unknown. The findings, however, do indicate a profound biological difference between men and women that may be relevant to sex ratios of disease.

Infectious Disease

General Principles Males do not differ from females in terms of their responses to infections, regardless of whether the invading organism is a virus, bacterium, mycobacterium, or parasite. In experiments with animals in controlled settings, males are more susceptible to parasites, fungi, bacteria, and viruses (Klein, 2000), probably because of hormonal effects. Most sex differences in humans, however, are caused by differences in exposures (a societal level effect) instead of differences between males and females at the individual, organ, or cell level. The following are examples of sex differences caused by different exposures:

• Exposure. Sex differences in attack rates occur with kuru (a disease caused by a prion), in which, for cultural reasons, only females eat the infected tissue; tuberculosis, when it is contracted in prison or homeless shelters; and infection with human immunodeficiency virus (HIV) when it is transmitted by male homosexual intercourse or needle sharing.
• Portal of entry. Differences in genitourinary anatomies and local immune responses cause different clinical phenotypes of gonorrhea and herpes genitalis.
• Organism load. Sex differences in organism loads are especially relevant in HIV infection; in part, these differences are also a function of portal of entry.
• Receptors. In experimental coxsackievirus myocarditis, male mouse hearts contain more viral receptors and receive higher viral loads.
• Social response. Clinical acknowledgment of illness, particularly in the face of stigma, may differ between males and females, as may compliance with therapy. Tuberculosis and leprosy in developing countries are examples.
• Response to therapy. Sex differences in response to therapy are discussed below in terms of drug metabolism.

Virus In humans, sex differences in viral illnesses are primarily due to differences in behavior, such as vaccination rates or exposure (Table 5–7). The apparent higher rate of fatality from measles in girls is not explained, and the different effects of vitamin A on antibody titers in males and females are controversial. In animals, sex differences of virus effects occur in both directions.

TABLE 5–7

Sex Differences in Viral Infections in Humans.

Bacteria Bacterial diseases affect males and females approximately equally. Even in the conversion of acute to chronic Lyme disease (an illness that closely resembles autoimmune disease), the incidence and severity in males and females are similar (Pena and Strickland, 1999).

Mycobacteria, Fungi, and Parasites Males animals are slightly more susceptible to infection with mycobacteria, fungi, and parasites (Klein, 2000). In humans, sex-specific rates of infection with mycobacteria, fungi, and parasites are approximately equal; most differences can be explained by differences in exposures (Table 5–8).

TABLE 5–8

Sex Differences in Mycobacterial, Fungal, and Parasitic Diseases.

Leprosy and Chagas' disease induce lupus autoantibodies and tuberculosis induces rheumatoid factor, but these diseases do not induce clinical autoimmune disease in humans. Data are scant, but infected males and females do not differ in their autoimmune responses to these diseases.

Autoimmune Disease

Definition, Classification, and Female/Male Ratios In autoimmunity the immune response is directed against host antigens instead of foreign invaders. The host antigens are either localized, as in thyroid and skin diseases, or ubiquitous, as in lupus. Autoimmunity characterizes the prototypical diseases whose occurrences differ by sex. Autoimmune diseases pose the central question for the study of such sex differences: what mechanisms explain discrepancies in disease occurrences by sex? Explaining sexual dimorphisms in autoimmune diseases will likely bring to light heretofore unknown important biological differences between females and males.

Autoimmunity is defined to occur when an antibody binds to or reacts with an autoantigen (an extract of a normal tissue). Cell-mediated mechanisms may participate in autoimmunity (Draca, 1995; Marchetti et al., 1998; Olff, 1999; Wilder, 1995). Other causes of autoimmunity include immunization and passive transfer of antibodies in animal models or participation of the major histocompatibility complex (MHC). There is no consensus definition, however. The different definitions and classifications partly explain why different diseases are named autoimmune diseases in standard medical texts. Most authorities agree that thyroid and rheumatic diseases are autoimmune diseases; they differ about inflammatory bowel disease, multiple sclerosis, some skin diseases, and juvenileonset diabetes.

Some autoimmune diseases are strikingly predominant in females, others are not predominant in either sex, and still others are predominant in males. Table 5–9 lists several such diseases. The female/male ratios vary 50-fold. Predominance in females applies to some, but not all, of these diseases.

TABLE 5–9

Female/Male Ratios Associated with Common Autoimmune Diseases.

The label “predominant in females” is commonly used for human illness and refers to sex differences in incidence. By contrast, in parallel diseases in experimental animals, the term often refers to differences in disease severity. In human autoimmune diseases, severity is similar in females and males.

Environmental Causes of Autoimmunity The likelihood that environmental factors (toxins and infections) induce autoimmune disease is supported by the diseases and circumstances listed in Table 5–10. In several exogenously induced mimics of autoimmune disease, sex differences in disease occurrence are caused by exposure differences. These diseases are described here.

TABLE 5–10

Autoimmune Diseases in Which Environmental Triggers Are Prominent.

Drug-induced lupus (Yung et al., 1997) and toxin-induced scleroderma-like disease (Abaitua Borda et al., 1998; Shulman, 1990) loosely resemble but are not identical to idiopathic autoimmune diseases, suggesting that exogenous agents cause idiopathic autoimmune disease. More males than females take drugs that induce lupus (male predominant), and more males are exposed to silica inducers of scleroderma-like disease (male predominant). In Spain it was found that more females were exposed to the contaminated cooking oil that causes a sclerodermalike illness (female predominant). More females than males took contaminated L-tryptophan, a putatively natural antidepressant; the resulting epidemic of eosinophilia-myalgia syndrome was predominant in females. Chronic Lyme disease is a self-perpetuating autoimmune illness that is initiated by but that does not require the persistence of live Borrelia burgdorferi organisms (Carlson et al., 2000). Its incidence has no sex difference, yet it closely resembles rheumatoid arthritis, whose incidence does have a sex difference. Fogo selvagem, or Brazilian endemic pemphigus foliaceus, is transmitted by a black fly bite and is presumed to be caused by an infectious agent passed at the time of the bite. Fogo selvagem shows no sex preference (Hans-Filho et al., 1999), but spontaneous pemphigus foliaceus, which occurs elsewhere in the world, is predominant in females. Contact dermatitis may be predominant in females, but differential exposure to allergens is the likely cause (Kwangsukstith and Maibach, 1995).

The possibility that infection induces rheumatic autoimmune disease is widely (Miller et al., 2000; Montgomery et al., 1999) but not universally (Ringrose, 1999) accepted. It is unknown how infections induce different diseases by sex (other than by different exposure rates). Potential male-female differences in the processing of infecting organisms, differences in vulnerable periods, or differences in threshold immune responses still apply.

Hormonal Causes of Autoimmunity, Including Life Events Case reports of clinical exacerbation or remission of autoimmune diseases after castration or hormone treatment suggest that gonadal hormone modulation plays an important role in disease severity in individuals but constitutes weak evidence for sexual dimorphisms in disease incidences in populations (Lahita, 1999). Studies of the effects of postmenopausal estrogen or oral contraceptive therapy on autoimmune disease incidence most often show that such therapy has little effect (Petri and Robinson, 1997). Synoviocyte estrogen receptors may be target organs in rheumatoid arthritis (Castagnetta et al., 1999), a possible explanation for the predominance of this illness in females. However, chronic Lyme disease causes a similar joint inflammation but is not predominant in females, and ankylosing spondylitis also causes a similar joint inflammation, but it is predominant in males. Androgens have no apparent role in ankylosing spondylitis (Giltay et al., 1999). Although experimental feminization worsens autoimmune diseases in animal models and experimental masculinization ameliorates autoimmune diseases in animal models, variations in both severity and incidence are found.

Rheumatoid arthritis goes into remission during pregnancy, contradicting the theory of estrogen-enhanced immunological activity. The remission of rheumatoid arthritis is likely due to a human leukocyte antigen (HLA) mismatch between mother and fetus rather than to pregnancy-associated hormones (Nelson et al., 1993; Ostensen, 1999). Multiple sclerosis also goes into remission during pregnancy (Confavreux et al., 1998). Although it is often cited that pregnancy induces the flare-up of lupus, lupus in fact does not worsen or worsens only slightly during pregnancy (Lockshin, 1993). Estrogen replacement therapy, oral contraceptives, and ovulation induction do not worsen lupus (Guballa et al., 2000).

Insight can be gained through the study of rheumatoid arthritis and multiple sclerosis through the study of pregnancy and events during gestation as well as during the postpartum period. In these two diseases, clinical symptoms frequently lessen substantially and can even abate during the third trimester of pregnancy, but the diseases flare soon after delivery.

At the cell level gonadal hormones modulate the immune response. Why and how (if it influences disease incidence at all) this effect influences disease incidence is unclear. Estrogen could play a permissive role, allowing survival of forbidden autoimmune clones. A threshold mechanism, that is, a specific level of estrogen at a vulnerable time, could explain the increase in incidence, but no such threshold has been postulated, tested, or demonstrated in humans. Hormones may also influence the frequency of autoimmune disease in males and females in ways that are independent of the immune system. It is possible that sex differences in the endothelium are critical for disease initiation. A still undiscovered sex difference related to, for example, ovulation- or menstruation-related cytokines, apoptosis of nonimmunological cells, or the presence of vaginal flora or the immune response to vaginal flora may be responsible for the different disease experiences of the two sexes.

Genetic Causes of Autoimmunity Evidence supporting the concept of genetic control of autoimmunity consists of studies with families and twins, the HLA types associated with specific illnesses, the identification of genes that enhance disease susceptibility, and transgenic experiments in which illness is induced in experimental animals (Seldin et al., 1990). Evidence of this type is particularly strong for spondyloarthropathy (Taurog et al., 1999), rheumatoid arthritis, and lupus.

HLA types by themselves do not explain the sexual dimorphism of the genetic causes of autoimmunity (Chen et al., 1999; Greenbaum et al., 2000), although sex differences in HLA-associated disease expression may occur (Lambert et al., 2000). For each haplotype associated with a sexually dimorphic autoimmune disease, there is another haplotype associated with an autoimmune disease that is not sexually dimorphic. For instance, HLA B27 is associated with both spondyloarthropathy (female/male ratio, 0.3) and uveitis (female/male ratio, 1.0), whereas DR3 is associated with Graves' disease (female/male ratio, 7.0), systemic lupus erythematosus (female/male ratio, 6.0), and myasthenia gravis (females/male ratio, 1.0).

With the exception of the CD40 ligand, few putative autoimmune markers identified to date are on the X or Y chromosome. No conclusive evidence for imprinting or X-chromosome inactivation differences exists for autoimmune diseases. The X chromosome has no role in human ankylosing spondylitis (Hoyle et al., 2000). Non-MHC genes may be relevant. In a mouse model of diabetes, mutation of a tissue or a developmental stage-specific proteasome product shows sex differences (Hayashi and Faustman, 1999). The sexual dimorphism of T-cell trafficking may be due to sex-determined cell surface markers or might be secondary to genomic or nongenomic effects.

Life Stage Causes of Autoimmunity Most diseases that are predominant in females cluster in the young-adult years, whereas autoimmune diseases that affect younger or older patients are more evenly divided between the sexes (Table 5–11).

TABLE 5–11

Peak Ages of Various Autoimmune Diseases.

Characteristics of young adulthood that may explain the predominance of a disease in females include the chronobiological effects of menstrual cycles, gonadal hormones, threshold effects, vascular responses, immune responses, vaginal flora, and other as yet unknown variables. Very little experimental work has considered these topics.

Animal Models of Autoimmunity Animal models of autoimmune disease give mixed messages about the causes of sex differences in autoimmunity. Table 5–12 displays some relevant data for three animal models of human autoimmune diseases, two of which are predominant in females and one of which is dominant in males. Some information is dated and can be challenged by new technology.

TABLE 5–12

Animal Models of Human Autoimmune Diseases.

Animal models of autoimmune disease use immunization, inbreeding, and transgenic and gene knockout methods. In the model of thyroiditis, different strains of mice and rats are variably susceptible, implying strong genetic control of this disease. Only young adult mice and rats were studied, however. In one rat strain, susceptibility was predominant in females. On the basis of backeross experiments, the X chromosome determines susceptibility. In mice, estrogen enhanced the antithyroid antibody titer (a marker of disease) but not thyroiditis itself. The severity of induced thyroiditis was also found to vary with diet. Thus, genetic, X-chromosome, hormonal, and extrinsic factors may influence the occurrence of thyroiditis.

In mouse models of spontaneous lupus, the incidence and severity of lupus are predominant in NZB×NZW (F1) strain females, whereas they are sex neutral in MRL lpr/lpr mice and are predominant in males of the BXSB strain. In animal models, spontaneous autoimmune lupus develops in young adulthood, implying that maturation or cumulative damage is required for disease expression. At maturation, susceptible mouse strains have more numerous and more avid estrogen receptors on lymphoid and uterine tissues than nonsusceptible strains, an explanation of strain susceptibility differences by strain but not of susceptibility differences by sex. Castration and replacement experiments demonstrate estrogen enhancement and testosterone suppression of spontaneous disease. When animals are raised in a germfree environment, however, neither the phenotype nor the disease incidence changes, except that females tend to have higher autoantibody levels than males. Raising animals in a germfree, antigenfree environment ameliorates disease. Males and females in germfree environments are affected equally. Gene knockout experiments give conflicting results. Of the two studies listed, one shows a markedly worse occurrence of glomerulonephritis in females, whereas the other shows equal incidences of glomerulonephritis in both sexes. Thus, in experimental lupus, genetic, hormonal, life stage, and environmental factors all appear to be relevant and sex differences remain unexplained.

The human HLA B27 gene transgenically expressed in rats induces a phenotype with features of psoriasis and ankylosing spondylitis. In a germfree environment, the spondylitis does not occur but the psoriasis does. Introduction of specific gastrointestinal pathogens to the germfree animal induces the spondylitis. The predominance of psoriasis and spondylitis in males is true of this model, as it is of the disease in humans. Genitourinary anatomy, sex hormones, immune response, and unknown factors are possible explanations.

In summary, animal models suggest that autoimmune diseases have specific genetic, hormonal, life stage, and environmental causes. The human sex differences are not reproduced in many of the animal models, but no attempt has been made to understand why.

Environment

The most important environmental agents influencing coronary heart disease are diet, drugs, airborne toxins, and, possibly, infectious agents; and the effects of these agents may be synergistic. For example, overeating or obesity and a sedentary lifestyle promote high blood pressure, high cholesterol, and diabetes, which are major risk factors for coronary heart disease in both males and females. Yet, susceptibilities and responses vary by sex.

Genetics

Dyslipidemias are among the strongest genetic contributors to coronary heart disease (Goldstein et al., 1973a,b; Hazzard et al., 1973). Familial hypercholesterolemia (FH) is a low-density lipoprotein (LDL) cholesterol disorder caused by a genetic defect of the LDL receptor on cells. It is autosomal dominant and is more severe in homozygotes because of a gene dosage effect (Goldstein et al., 1995).

Homozygotic defects occur in about 1 in 1 million individuals, and such individuals present with cholesterol levels that range from 650 to 1,000 milligrams per deciliter (mg/dl); heterozygotic defects occur in about 1 in 500 individuals, and such individuals present with cholesterol levels that range from 350 to 550 mg/dl (Goldstein et al., 1995). Homozygotes develop atherosclerosis, which leads to coronary heart disease and death usually before age 20. Sex differences are undocumented in homozygotes but do occur among heterozygotes; heterozygous males die about 10 years earlier than heterozygous females (Table 5–13). In the FH heterozygote population, the age differences in incidence between the sexes are comparable to those for the general population.

TABLE 5–13

Estimated Risk of Symptoms of Coronary Heart Disease and Death from Myocardial Infarction in Heterozygotes at Different Ages.

Age

Sex differences in heart disease mortality occur over the life span between women and men (Figure 5–3). The Barker hypothesis postulates that the genesis of coronary heart disease may begin in utero, perhaps in response to malnutrition and stress (Barker, 2000; see also Chapter 3). The fetus may adapt to the lack of critical nutrients or an excess of maternal stress hormones by permanently altering metabolic, endocrine, and cardiovascular systems in such a manner as to promote atherosclerosis later in life (Barker, 1999).

FIGURE 5–3

Death rates for diseases of the heart by age and sex, 1995–1997. Source: National Center for Health Statistics (1999).

Several longitudinal studies have demonstrated sex differences in coronary heart disease. The Framingham Heart Study, initiated in 1948 (Dawber et al., 1951), demonstrated that cardiovascular differences exist between the sexes. For example, it established the classical risk factor concept for coronary heart disease. This landmark study showed that a raised serum total cholesterol level, high blood pressure (systolic and diastolic), and smoking increase the risk of developing coronary heart disease in men and women in a graded fashion. Women develop coronary heart disease about 10 years later than men and women's risk is smaller.

The Bogalusa Heart Study followed African-American and Caucasian children over time, beginning in the 1960s, demonstrating that sex differences in risks for coronary heart disease begin at an early age.

The longitudinal Tromso Heart Study (Norway) (Stensland-Bugge et al., 2000) reexamined 3,000 middle-aged women and men in Norway 15 years after an initial examination. Hypertension, total cholesterol levels, high-density lipoprotein (HDL) cholesterol levels, and body mass index independently predicted increased carotid intimal thickness in both women and men. However, triglyceride levels were an independent risk factor in women but not men, whereas physical activity and smoking were independent risk factors in men but not women.

The National Health and Nutrition Examination Survey is a federal epidemiological follow-up study that has collected national data on U.S. residents since the 1960s and that provides evidence of the incidence and prevalence of a variety of health indicators by age, race, and sex (National Center for Health Statistics, 2000b). These data again show differences in cardiovascular disease incidence between the sexes and among racial and ethnic groups. For example, the age-adjusted risk for incident coronary heart disease is higher in African-American women ages 20 to 54 years than in Caucasian women of the same age and lower in African-American men than in Caucasian men of the same age (Gillum et al., 1997).

In terms of risk factors, men have higher prevalences of hypertension, cigarette smoking (until age 65), and excess weight, whereas women have higher prevalences of elevated serum cholesterol levels and obesity (National Center for Health Statistics, 2000a). Regarding the age-specific prevalences of hypertension and serum cholesterol levels greater than 240 mg/dl, men have a higher prevalence than women until about their late 40s and early 50s. After that, the prevalence is higher in women.

These studies underscore the fact that coronary heart disease begins early in life, that it continues across the life span, and that sex differences exist. What they do not demonstrate is why such differences exist.

Cigarette Smoking

Many airborne toxins that are absorbed through the respiratory system predispose an individual to the development of coronary heart disease. One of the most pervasive and lethal of these agents is cigarette smoke.

Cigarette smoking is more frequent among men than women, especially in African Americans (National Center for Health Statistics, 1999) (Table 5–14).

TABLE 5–14

Smoking Prevalence by Race and Sex, 1998.

Data from the Centers for Disease Control and Prevention indicate that both men and women who smoke have three times the risk of dying from heart disease as individuals who do not smoke. In women, smoking (Baron et al., 1988; Michnovicz et al., 1986) promotes susceptibility to early menopause and to a number of diseases, including coronary heart disease.

Cholesterol

Sex differences exist in the levels of lipoprotein subfractions of cholesterol: LDL cholesterol, HDL cholesterol, and very-low-density lipoprotein (VLDL) cholesterol (Figure 5–4 and 5–5).

FIGURE 5–4

Age-adjusted high serum cholesterol levels (>240 mg/dl) among individuals ages 20 to 74 years, by sex and race, 1988–1994. Source: National Center for Health Statistics (1999).

FIGURE 5–5

Non-age-adjusted high serum cholesterol levels (>240 mg/dl) by sex and age. Source: National Center for Health Statistics (1999).

Before menopause, women have higher HDL cholesterol (“good cholesterol”) levels and lower LDL cholesterol (“bad cholesterol”) levels. During the peri- and postmenopausal periods, however, LDL cholesterol levels rise and HDL cholesterol levels drop. Rates of death from heart disease rise with age in both men and women, but the rate of ascent rises more sharply in women as menopause ensues.

Estrogen affords women a protective advantage against coronary heart disease before menopause. 17-β-Estradiol increases HDL cholesterol levels and decreases LDL cholesterol levels, stimulates nitric oxide, and inhibits vasoconstricting factors (Collins, 2000). These factors may operate independently or synergistically (Collins, 2000). Estrogen also prevents calcium influx through ion channels in the membranes of vascular smooth muscle cells, preventing vessel contraction.

During perimenopause estrogen levels begin to decline and continue to do so through menopause, and HDL cholesterol levels fall as LDL cholesterol levels rise. Hepatic LDL cholesterol receptor activity may explain these changes (Semenkovich and Ostlund, 1987). Another explanation for the increased incidence of cardiovascular disease after menopause is that the LDL cholesterol particles may become denser and therefore less protective (Campos et al., 1988; Haffner et al., 1993).

High triglyceride levels present a greater risk to women than to men. HDL cholesterol levels may be a better predictor than LDL cholesterol levels of coronary heart disease risk in women (Gordon et al., 1989; Jacobs et al., 1990).

As noted above, endogenous estrogen present in premenopausal women appears to have a cardioprotective effect. Observational studies of postmenopausal women taking hormone replacement therapy, and experimental studies with animals, demonstrate that unopposed estrogen has a strong effect in preventing atherosclerosis and its clinical sequelae (Barrett-Connor, 1998a; Barrett-Connor and Grady, 1998). These studies however have not been supported in two large clinical trials of estrogen combined with progestin in women with established coronary disease (Hulley et al., 1998; Herrington et al., 2000). Other clinical trials including women both with and without established coronary disease are currently in progress. As a result, the issue of whether hormone replacement therapy has cardioprotective effects must be considered unresolved.

Hypertension

The causes of hypertension are unknown (Williams, 1991), yet studies suggest multiple factors, polygenetic and environmental. Essential hypertension is the most common form, and it affects a substantial portion of the U.S. population. Risk factors for hypertension include age, sex, smoking, diet, elevated blood cholesterol levels, obesity, diabetes, sedentary lifestyle, and family history.

Hypertension is both a risk factor for coronary heart disease and a disease itself. As a result of hypertension, the heart wall thickens and its function declines. There are differences in the association of hypertension and coronary heart disease by sex (Fiebach et al., 1989; Kannel et al., 1976; Sigurdsson et al., 1984) and race (Johnson et al., 1986).

Men have higher blood pressure levels than women (National Center for Health Statistics, 2000a, p. 245). Height differences as well as differences in mass contribute to differences in blood pressure between men and women. The genesis of hypertension in adulthood often occurs in childhood, and weight is a greater predictor for hypertension in girls than in boys (Cook et al., 1997).

Blood pressure is higher during the follicular phase than during the luteal phase of the menstrual cycle in both normotensive and hypertensive women (Dunne et al., 1991). African-American women respond to stress with a higher diastolic pressure and higher plasma epinephrine levels during the follicular phase than during the luteal phase, but Caucasian women (and men of all ethnicities) do not exhibit any significant change in blood pressure over the course of a month (Ahwal et al., 1997; Mills et al., 1996). These results have been disputed by investigators (Litschauer et al., 1998), who believe that the differences may be caused by an interaction of sex and task characteristics (cause of stress).

The angiotensin-converting enzyme deletion-insertion polymorphism appears to be associated with systemic hypertension in Caucasian men (O'Donnell et al., 1998). In Japanese men but not Japanese women there is a similar association between angiotensin-converting enzyme gene polymorphism and hypertension (Higaki et al., 2000).

Two hypertension-associated conditions are sex specific: preeclampsia and hypertension of pregnancy. Both conditions are characterized by increasing levels of hypertension as pregnancy progresses and are most common in the last trimester. Both are serious and can be fatal. Whether either of these conditions progresses to coronary heart disease is not understood, although two studies have shown that such a relationship exists (Croft and Hannaford, 1989; Rosenberg et al., 1983).

Diabetes Mellitus

Diabetes mellitus is a risk factor for coronary heart disease and is an example of the sex differences in the risk for coronary heart disease. Premenopausal women, who are not typically at risk for coronary heart disease, are at risk if they have type I (juvenile) or type II (adult-onset, noninsulin-dependent) diabetes mellitus (Figure 5–6).

FIGURE 5–6

Mortality from coronary heart disease and diabetes in men and women ages 25 to 64. Source: Krolewski et al. (1991). Reprinted, with permission, from A.S.Krolewski, J.H.Warram, P.Valsania, B.C.Martin, L.M.Laffel, and A. R.Christlieb. 1991. Evolving natural (more...)

Rates of mortality from coronary heart disease are two to four times greater in diabetic men than nondiabetic men and three to seven times greater in diabetic women than nondiabetic women (Barrett-Connor and Wingard, 1983; Kannel and Abbott, 1987; Manson et al., 1991; Pan et al., 1986). Diabetes may negate estrogen receptor binding and thereby mitigate the positive estrogenic effect (Ruderman and Haudenschild, 1984).

Diabetes of pregnancy may represent a risk for the development of non-insulin-dependent diabetes mellitus after pregnancy. Current evidence suggests that gestational diabetes is a risk factor for coronary heart disease (Mestman, 1988; O'Sullivan, 1984; Stowers, 1984).