Leucovorin (Folinic Acid)

LV serves as donor of the methyl group during pyrimidine synthesis by TS. In preclinical models, LV modulates 5-FU antitumor activity both in vitro and in vivo. LV is a racemic mixture, half of which exist in l isomer (active) form. To achieve maximal modulation, LV is given as bolus injection mid of 1-h 5-FU infusion. Oral LV is also available and effective. Randomized studies have established superiority of 5-FU/LV over 5-FU in objective response rate median progression-free survival and the overall survival.^395,396 These results have been confirmed by meta-analysis.³⁹⁷ There is no difference in the antitumor efficacy between the Mayo Clinic regimen and the Roswell Park regimen. The toxicity profile varies and the Roswell Park regimen resulted in more hospitalizations as compared to the Mayo Clinic regime.³⁹⁸

Continuous Infusion 5-FU

In view of the short plasma half-life of 5-FU, most experts believe that CI5FU is the optimal 5-FU schedule. Continuous infusion 5-FU promotes the polyglutamylations of leucovorin within the tumor cell, thereby improving the bioavailability of LV in the target cell. Several randomized studies comparing CIFU versus bolus 5-FU with or without LV suggest the superiority of CIFU over bolus 5-FU in terms of improved response rate and reduced toxicity profile. However, no improvements in survival were noted. A meta-analysis from 7 randomized studies with a total of 1,219 patients confirmed superior response rate of CIFU over bolus 5-FU (22% vs 11%) and a small but statistically significant survival advantage (12.4 months vs 11.4 months p = .02) over bolus 5-FU/LV.³⁹⁹ With continuous infusion 5-FU there was a higher incidence of the hand-and-foot syndrome, but a lower incidence of neutropenia, diarrhea, and mucositis.

Chronomodulation

Circadian variations in the enzymatic activities of DPD and TS have been demonstrated.⁴⁰⁰ Dihydropyrimidine dehydrogenase activity in human mononuclear cells increases by 50% around midnight and, therefore, theoretically, contributes to better tolerance of 5-FU if the latter is administered before midnight and 4 a.m.⁴⁰⁰ In a study comparing standard versus chronomodulated 5-FU infusion with oxaliplatin without a 5-FU standard arm, the response rate in both previously treated and untreated patients seemed to favor the chronomodulated arm. There was a higher response rate, median survival, and lower toxicity profile with chronomodulated therapy.⁴⁰¹ However, because of the lack of a control arm with 5-FU/LV, interpretation of the results must be made cautiously.

Methotrexate

Methotrexate (MTX) modulates 5-FU cytotoxicity by increasing the intracell content of phosphoribosyl pyrophosphate (PRPP), which, in turn, decreases the purine production, with the result that thymidylate biosynthesis is upregulated, increasing incorporation of 5-FU into RNA or DNA. A 7- to 24-h interval in the timing of 5-FU following MTX appears to be critical for the antitumor activity. In Phase II trials, 5-FU/LV was proven to be superior to 5-FU/MTX.^396,398 A meta-analysis evaluating the biochemical modulation of 5-FU by MTX revealed a very modest gain in overall survival in patients treated with 5-FU/MTX as compared to those treated with 5-FU alone.⁴⁰² The 5-FU/MTX combination is rarely used in clinical practice.

Trimetrexate

Trimetrexate (TMX) is a synthetic inhibitor of dihydrofolate reductase that differs from MTX in several important aspects. In contrast to MTX, TMX is more lipophilic, does not require activation by folypolyglutamyl synthase, and does not use the reduced folate transporter system to enter neoplastic cells, thus avoiding competition for uptake when administered together with folinic acid. TMX as a single agent is associated with a 20% response rate.⁴⁰³ In previously untreated patients in Phase II studies, the combination of TMX plus 5-FU/LV produced response rates of 36% and 42%.^404,405 In these studies, grades 3 and 4 diarrhea occurred in 58% of the patients. Preliminary results from a Phase III randomized trial have revealed no advantage of 5-FU/LV/TMX over 5-FU/LV.⁴⁰⁶

Interferon-α2a

The use of IFN-α2a was based on in vitro findings that IFN-α2a synergizes with 5-FU cytotoxicity in a dose- and schedule-dependent manner. Early studies of 5-FU with IFN-α2a suggested a greater than 60% response rate in previously untreated patients.⁴⁰⁷ However, those single-institution results were not reproduced. Toxicity with the latter combination is significant.

Zidovudine

Preclinical models showed that de novo inhibition of TS by 5-FU increased the cytotoxicity of zidovudine (AZT) against human colorectal cell lines. An objective response rate of 44% was reported in a small clinical study of 5-FU plus AZT in chemo-naïve patients.⁴⁰⁸

N-Phosphonoacetyl-i-Aspartic Acid

Small-er studies showed that N-phosphonoacetyl-l-aspartic acid (PALA) combined with 5-FU increased 5-FU activity; larger randomized studies, however, failed to confirm this claim.^409,410

Modulation of 5-FU infusional schedules was attempted in the seven-arm Southwest Oncology Group (SWOG) randomized Phase II study.⁴¹¹ The study compared bolus 5-FU with or without LV, CIFU with or without LV, and 24-h infusion of 5-FU with or without PALA. Both response and median survival rates were very comparable among all arms.⁴¹¹

Hepatic Artery Infusion

The liver is the most common site of visceral metastases from colon cancer. Approximately 25% of the patients will have metastatic disease confined to the liver at presentation. If the disease is not resectable, chemotherapy may be appropriate. Intrahepatic arterial therapy of FUDR is based on the fact that metastases derive their blood supply almost exclusively via the hepatic artery, whereas normal hepatocytes derive most of their blood supply from portal circulation. Floxuridine has an extremely short systemic half-life of only a few seconds. In fact, 94% to 99% of floxuridine is extracted during the first pass through the hepatic arterial circulation as compared to 19% to 55% of 5-FU, thus making floxuridine an ideal candidate for intrahepatic therapy because it can achieve high intrahepatic drug levels without high systemic drug levels.⁴¹² HAI therapy involves placement of either percutaneous catheter, implantable pumps, or external pumps, all of which require surgery and hospitalization. Ideal candidates for HAI should not have any evidence of extrahepatic disease. Complications of HAI include biliary sclerosis, chemical hepatitis, gastritis, and peptic ulcer.⁴¹²

Several investigators have published encouraging response rates with HAI. In a meta-analysis of HAI, there was a survival difference in favor of HAI versus no HAI, but the survival benefits were absent when two studies with suboptimal systemic chemotherapy were excluded.⁴¹³ The lack of effective systemic effect with HAI therapy may be the explanation for the failure of HAI to translate the intrahepatic response rates into improved overall survival. Current efforts are directed at improving the systemic control by combining HAI with either oxaliplatin or irinotecan, or with biologic agents.⁴¹² The use of radioactive yttrium spheres to deliver radiation to the liver has been investigated.⁴¹⁴ One study revealed that SIR-spheres-plus-HAI-treated patients had an overall better response rate (44% vs 17%; p = .01), median time to progression (15.9 months vs 9.7 months) and an improved survival when compared to patients treated with HAI alone.⁴¹⁴

The advent of effective systemic chemotherapy agents and other locoregional measures has made HAI a less-favorable alternative in the treatment of unresectable isolated hepatic metastases. HAI in the adjuvant setting was discussed in a previous section.

Capecitabine (Xeloda)®

Capecitabine, a 5-FU prodrug, is a rationally designed tumor-activated and potentially tumor-selective fluoropyrimidine carbamate. After oral administration, capecitabine, a 5′-deoxy-5-fluorocytidine (5′-DFCR) derivative, is absorbed unchanged from the gastrointestinal tract and is metabolized in the liver by carboxylesterase to 5′-DFCR. 5′-DFCR is then converted to 5′-DFUR by cytidine deaminase located in the liver and in tumor tissues.⁴¹⁶ Further metabolism of 5′-DFUR to 5′-FU then occurs at the site of the tumor under the action of thymidine phosphorylase, where it is upregulated compared to normal tissue. As a result, the 5-FU concentration ratio with capecitabine is 3-fold higher in the tumor than in adjacent colon mucosa and 22-fold higher in the tumor than in plasma in contrast to a 1:1:1 concentration ratio with infusional 5-FU.⁴¹⁶

Based on Phase I study results, dosing schedules selected for Phase II development included continuous daily dosing (day 1 to 14 schedules every 3 weeks) or intermittent dosing with leucovorin. The intermittent capecitabine schedule (1,250 mg/m² daily × 14 days every 21 days) was selected for Phase III development. Capecitabine major toxicities include diarrhea and hand-and-foot syndrome.

Two large randomized Phase III studies compared capecitabine to the Mayo Clinic 5-FU/LV as first-line therapy in metastatic colorectal cancer.^417,418 Both studies reported an improved overall objective tumor response rate with capecitabine, but this response rate was not associated with an increase in median time to tumor progression or median overall survival.^417,418 Capecitabine produced lower incidences of nausea, stomatitis, alopecia, and neutropenic fever/sepsis, but increased incidence of hand-and-foot syndrome similar to the toxic effects of CIFU. The etiology of the hand-and-foot syndrome is unknown, but a recent report suggests that it is partly a result of an inflammatory process because it can be attenuated by celecoxib.⁴¹⁹

UFT

UFT consists of 1:4 mixture of uracil and tegafur. The latter is a prodrug of 5-FU and the former is a reversible, competitive inhibitor of DPD, thereby increasing the bioavailability of 5-FU.⁴²⁰ Two large Phase III randomized studies were performed in patients with metastatic colorectal cancer, comparing UFT with the Mayo Clinic schedule 5-FU/LV.^421,422 Both studies demonstrated equivalency in overall tumor response, time to tumor progression, and median survival. Diarrhea, nausea, vomiting, stomatitis, and mucositis were significantly less frequent with UFT/LV, as were myelosuppression, and febrile neutropenia.

5-FU and Eniluracil

DPD activity is inversely proportional to 5-FU response. Eniluracil is an irreversible DPD inhibitor that is safe when administered with oral 5-FU at very low dosages. Compared with intravenous 5-FU/LV, oral 5/FU with eniluracil was modestly inferior in both overall survival and objective tumor response.⁴²³

Irinotecan (Camptothecin, CPT-11)

Camptothecin is a topoisomerase I inhibitor derived from Camptotheca accuminata, a tree native to China. Topoisomerase I is an enzyme that produces single-strand breaks allowing the uncoiling of the supercoiled DNA, a prerequisite for DNA replication or DNA repair.⁴²⁴ Early clinical trials with camptothecin were suspended because of unacceptably severe myelosuppression and hemorrhagic cystitis attributable to the active lactone ring in camptothecin.⁴²⁴ However, irinotecan (CPT-11), a water-soluble precursor of the lipophilic metabolite SN-38 formed by carboxylesterase-mediated cleavage of the carbamate bond linking the camptothecin moiety to the dipiperidino side chain, was developed and studied.⁴²⁴ Irinotecan and SN-38 exist in a pH-dependent equilibrium between an active lactone form and an inactive hydroxyl acid form. In vitro, SN-38 is 1,000 times more potent than irinotecan as a topoisomerase I inhibitor; however, the percent contribution of SN-38 to the clinical activity or toxicity of irinotecan is not known.⁴²⁴

As a single agent, irinotecan produced consistently 10% to 20% objective response rates in newly diagnosed and in previously fluorouracil-treated patients with metastatic colorectal cancer.⁴²⁵ Irinotecan is given over 30 or 90 min either every 3 weeks (250 to 350 mg/m²) or weekly (100 mg/m² to 125 mg/m²) for 4 weeks with 2-week rest periods. It can produce an acute cholinergic reaction during or shortly after infusion, consisting of nausea, vomiting, acute salivation, facial flushing, diaphoresis, abdominal cramps, and diarrhea. This acute reaction can be quickly reversed with atropine and discontinuation of the infusion. The delayed irinotecan toxic effect is the cholera-like diarrhea, which occurs 7 to 10 days after the irinotecan administration. The persistent explosive watery stools can lead to severe dehydration with vascular collapse, shock, and death. Weekly infusion of irinotecan is associated up to 35% to 50% grades III to IV diarrhea. Other side effects include neutropenia, alopecia, asthenia, and hepatic toxicity. The elimination of CPT-11 and SN-38 is biphasic. The first phase is primarily in the urine and the bowel, followed by an intrahepatic recycling phase with the additional excretion in saliva and sweat.⁴²⁴ It is important to note that patients with Gilbert syndrome and other liver dysfunctions require major dose reductions of irinotecan.

Studies in Europe and in the United States demonstrate the benefit of CTP-11. In the United Kingdom, in one study, 287 patients were randomized in a 2:1 ratio to irinotecan plus best supportive care (BSC) versus BSC in patients with metastatic disease.⁴²⁶ There was a 41% improvement in median overall survival (9.2 vs 6.5 months) and better quality of life scores in patients randomized to irinotecan plus BSC arm.⁴²⁶ In France, Rougier and colleagues randomized 256 patients in 1:1 ratio to either irinotecan or retreatment with 5-FU and reported superior median survival in the irinotecan group (10.8 months vs 8.5 months).⁴²⁷

Phase III studies were conducted to evaluate Irinotecan combined with or without standard 5-FU/LV. In the United States, Saltz and colleagues compared 5-FU/LV (Mayo Clinic regimen) to IFL (5-FU 500 mg/m², LV 20 mg/m², irinotecan 125 mg/m² weekly for 4 weeks every 6 weeks; Saltz regimen) and to irinotecan alone (125 mg/m² weekly for 4 weeks every 6 weeks).⁴²⁸ The three-drug regimen was superior to 5-FU/LV or to CPT-11 alone; the latter produced similar results as the two-drug regimen. The median progression free-survival (7.0 vs 4.3 months) and the median overall survival (14.8 vs 12.6 months) were statistically significant in the three-drug regimen as compared to standard 5-FU/LV.⁴²⁷ In an European multiinstitutional study in which patients were randomized to either 5-FU/LV/irinotecan or 5-FU/LV, similar results were obtained with the median progression-free survival being 6.7 months vs 4.4 months and a median survival of 17.4 months vs 14.1 months in the three-drug regimen compared to the two-drug regimen, respectively.⁴²⁹ The three-drug regimen is not innocuous. Toxicity can be severe, with diarrhea and neutropenia being the most common side effects. The Saltz regimen has been associated with an increased mortality.³⁸¹ It is important to note that irinotecan administration must precede 5-FU administration, as reversal of the administration sequence will lead to reductions in SN-38 levels.

Oxaliplatin

Oxaliplatin, a diaminocyclohexane platinum derivative, binds to DNA, leading to DNA adduct formation subsequently causing irreversible DNA replication error and apoptosis.⁴³⁰ As a single agent, oxaliplatin reportedly has a 10% overall response rate in metastatic disease.⁴³⁰ As a second-line therapy in patients who have failed 5-FU/LV, oxaliplatin in combination with 5-FU/LV results in objective responses of 15% to 25%.⁴³⁰ In previously untreated patients, 5-FU/LV plus oxaliplatin gives a response rate of 40% to 50%.^430,431

The combination of 5-FU/LV/oxaliplatin can be administered with different 5-FU/LV regimens. However, the majority of Phase III studies have been conducted with infusional 5-FU regimens (5-fluorouracil, leucovorin, oxaliplatin [FOLFOX]). With chronomodulated 5-FU, the response rates have consistently improved and the toxicity has decreased.⁴³² Two randomized Phase III studies comparing infusional 5-FU/LV with or without oxaliplatin revealed that FOLFOX was superior in overall response rate (51% and 53% vs 16% and 22%) and progression-free survival (8.7 months and 9 months vs 6.1 and 6.2 months) to 5-FU/LV.^432,433 The lack of overall survival difference in both studies was attributable to crossover and to insufficient statistical power in the studies. Diarrhea, hematologic toxicity, and peripheral neuropathy are the most common toxicities of FOLFOX.

Oxaliplatin is being studied in combination with irinotecan (OXIRI) or with CIFU/LV and irinotecan (FOLFIRI). The Intergroup INT 9741 study randomized patients with metastatic colorectal cancer to IFL (Saltz regimen) versus infusion FOLFOX or to OXIRI.⁴³⁵ In that study, excessive deaths within 60 days were encountered in the IFL arm and the trial was stopped. After IFL dose modification and elimination of the OXIRI arm, the trial reopened.⁴³⁵ The most common grade 3 or higher toxicities were neutropenia and diarrhea in the IFL and OXIRI arms and neutropenia in the FOLFOX arm. There was a superior response rate (38% vs 28%), improved time to tumor progression (6.9 months to 8.8 months), and increased median overall survival (14 months to 18.8 months) in favor of the FOLFOX arm when compared to the IFL regimen.⁴³⁵ The use of the CIFU/LV infusion schedule used in the FOLFOX arm of the study appeared to have contributed, at least in part, to its improved tolerability and efficacy over the bolus 5-FU/LV used in IFL. A study of 215 patients with metastatic colorectal cancer evaluating FOLFIRI versus FOLFOX demonstrated equivalency in both arms the overall tumor response, time-to-tumor progression, and overall survival in the first- and second-line setting.⁴³⁶ In that study, crossover at the time of tumor progression was allowed.

Recently, in the United States, oxaliplatin in combination with 5-FU/LV was approved for metastatic colorectal cancer after failure of first-line chemotherapy. The basis of this decision was an ongoing Phase III multicenter trial conducted in the United States and Canada in which patients were randomized to either infusional 5-FU/LV, oxaliplatin alone, or infusional 5-FU/LV plus oxaliplatin.⁴³⁶ In the trial, response rate and time-to-tumor progression were statistically significantly improved in the combined infusional 5-FU/LV plus oxaliplatin arm.⁴³⁶

Oxaliplatin is not associated with increased renal toxicity. The major toxicities are bone marrow suppression and peripheral neuropathy. As compared with IFL, FOLFOX is very well tolerated with a favorable toxic profile. The incidence of grades 3 or 4 diarrhea, nausea, and vomiting is approximately 10%. The neuropathy is cumulative after oxaliplatin infusion, and generally occurs after six cycles. The neuropathy is generally reversible.

Raltitrexed (Tomudex)

Tomudex, a specific thymidylate synthase inhibitor, may be another alternative to 5-FU/LV. In three of four Phase III trials, median survival with raltitrexed was comparable to that of bolus of infusional 5-FU/LV.⁴³⁷ The most common toxicities of raltitrexed were gastrointestinal and hematologic toxicities.⁴³⁷

A Phase I multicenter study evaluating irinotecan plus raltitrexed was recently reported.⁴³⁹ In 13 patients, there was a response rate of 17%. Diarrhea was the dose-limiting toxicity, but was not more common than what has been reported with irinotecan alone. The combination of raltitrexed and oxaliplatin has been evaluated and is feasible. Further studies are warranted.⁴⁴⁰

S-1

S-1, a newly developed oral anticancer drug, is a combination of tegafur, gimestat (CDHP), a DPD inhibitor, and otastat potassium (Oxo), an inhibitor of orotate pyrimidine phosphoribosyl transferase.⁴⁴¹ The latter two compounds were combined with tegafur in an attempt to modulate the 5-FU toxicity.⁴⁴¹ In a Phase II study, the response rate of metastatic colorectal cancer to S-1 was 35%.⁴⁴¹ Grades 3 to 4 neutropenia occurred in 13% of the patients whereas other grade 3 to 4 toxicities occurred in fewer than 10% of patients.⁴⁴¹

Epidermal Growth Factor

Overexpression of epidermal growth factor (EGF) receptor has been noted in colorectal cancer. In fact, in one study it was reported that the level of expression of EGF receptor in colorectal cancer was inversely associated with mortality.⁴⁴⁶ Erbitux C225 is a monoclonal antibody developed to target the EGF receptor (EGFR). As single agent, it appears to have modest antitumor activity in colorectal cancer, but appears to have synergistic effects when combined with cytotoxic chemotherapy.⁴⁴⁶ Retreatment in patients who are irinotecan refractory with irinotecan plus C225 reportedly produces a response rate of 22.5%, with another 31% of the patients experiencing disease stabilization.⁴⁴⁷

Targeting protein kinases in the EGF receptor is another novel approach being evaluated. Agents such as Ires'sa (ZD1839), a pharmacologic inhibitor of EGFR tyrosine kinase, and PKI116, a dual inhibitor of EGFR/erb-2 tyrosine kinesis are being pursued.^446,448

Vascular Endothelial Growth Factor

Vascular endothelial growth factor (VEGF) is a vascular proliferation and permeability regulator. VEGF is overexpressed in colorectal cancer. The biologic effects of VEGF are mediated by two receptor tyrosine kinases (VEGFR-1 and VEGFR-2).^449,450 VEGFR-2 is thought to be the dominant signal transduction pathway regulating angiogenesis.⁴⁵⁰ Based on activity in preclinical models leading to tumor regression, VEGF inhibitors have been taken to the clinical arena. RhuMab-VEGF bevacizumab (Avastin), a selective inhibitor of VEGFR-2, has been evaluated alone or in combination with 5-FU/LV. In a Phase II study, the response rate of 5-FU/LV plus low-dose bevacizumab was 40%, compared to 17% in the chemotherapy alone group.⁴⁵¹ Fever and chills, headaches, hypertension, infection, rash, and nose bleeds were more common in the antibody-treated groups.⁴⁵¹

SU5416, another inhibitor of VEGFR-2 inhibitor was withdrawn from development after the interim analysis of a Phase I/II study suggested no statistical probability of superiority of IFL plus SU5416 over IFL alone.⁴⁵² Other trials with similar drugs are being pursued.

Cyclooxygenase-2

Over expression of cyclooxygenase-2 (COX-2), has been noted in 70% to 80% of colorectal cancer patients and is associated with angiogenesis, chemotherapy resistance, immunosuppression, tumor growth, and apoptosis evasion.^453,454 Randomized prospective clinical trials integrating celecoxib with cytotoxic chemotherapy have been launched.