Background

[PubMed]

One of the mechanisms by which cells mitigate the cytotoxic effects of chemotherapeutic agents, such as Adriamycin, Vinca alkaloids, epipodophyllotoxins, actinomycin D, and Paclitaxel, is to limit their presence inside the cells via the transmembrane protein P-glycoprotein (P-gp), which is encoded by the multidrug resistance (MDR-1) gene (1, 2). P-gp is an ATP-dependent multidrug transporter that is capable of actively pumping a variety of agents out of cells. Injection of unlabeled efflux pump substrates increases retention of radioactivity in the tumor rather than lessen the radioactivity, as seen with receptor-binding radiotracers. Overexpression of P-gp in tumor cells (such as renal carcinoma, hepatoma, pheochromocytoma, and colon carcinoma) leads to resistance to anticancer drugs (3). P-gp is also present in a variety of normal cells, such as intestinal mucosal cells, hepatocytes, renal proximal tubule epithelial cells, and endothelial cells of the blood-brain barrier (BBB) (4). Calcium channel blockers, cyclosporin A, and its non-immunosuppressive analog PSC 833 are MDR modulators, inhibiting transport of P-gp substrates out of the cells (5, 6).

Sestamibi (MIBI) is a substrate for P-gp, ^99mTc-MIBI has been approved by the U.S. Food and Drug Administration as a myocardial perfusion imaging agent with single-photon emission computed tomography (SPECT) to assess the risk of future cardiac events. It is also used as a tumor-imaging agent in breast, lung, thyroid, and brain cancers. Verapamil, a calcium channel blocker, is also a transport substrate for the P-gp efflux pump (7). [¹¹C]Verapamil is being developed as a positron emission tomography (PET) agent for studying P-gp function non-invasively.

Synthesis

[PubMed]

In their published method, Elsinga et al. (8) synthesized [¹¹C]verapamil by alkylation of desmethylverapamil with [¹¹C]methyl iodide. After high-performance liquid chromatographic separation, radiochemical yield was 16%, based on [¹¹C]methyl iodide. Total synthesis time was 45 min, and radiochemical purity was >99%. The specific activity was >11.1 TBq/mmol (300 Ci/mmol) at end of synthesis (EOS).

Wegman et al. (9) described an improved method for preparing [¹¹C]verapamil by reaction of [¹¹C]methyltriflate with desmethylverapamil. [¹¹C]Verapamil was prepared with a reproducible radiochemical yield of 66 ± 4% (end of bombardment (EOB)) based on [¹¹C]methyltriflate. Total synthesis time was 60 min. Radiochemical purity was >99%, with specific activities varying between 5 and 30 TBq/mmol (135-1110 Ci/mmol) at EOS.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

In the study by Elsinga et al. (8), cellular accumulation of [¹¹C]verapamil was 3.5-fold higher (345 ± 102 pmol/10⁶ cells) in the human ovarian carcinoma cell line A2780 than in the corresponding Adriamycin-resistant cell line 2780AD (77 ± 15 pmol/10⁶ cells). 2780AD cells were found to have higher P-gp expression than the A2780 cells. In a study comparing a P-gp-negative small cell lung carcinoma (GLC₄) and its overexpressing subclone (GLC₄/P-gp), cellular accumulation of [¹¹C]verapamil in GLC₄ was 1.3-fold higher than in GLC₄/P-gp (10). Cyclosporin A increased [¹¹C]verapamil accumulation by 1.0-fold in GLC₄/P-gp but had no effect in GLC₄.

Animal Studies

Human Studies

[PubMed]

Hendrikse et al. (15) reported on PET studies in 5 cancer patients after injection of 370 MBq (10 mCi) of [¹¹C]verapamil. Accumulation of radioactivity was 43.0%, 1.3%, and 0.9% ID in the lungs, heart, and tumor, respectively. From the time-activity curves, the pharmacokinetic half-life of [¹¹C]verapamil was 46.2, 73.8, and 23.7 min in the lungs, heart, and tumor, respectively. The efflux of [¹¹C]verapamil out of tumors was relatively fast.

In one published study, [¹¹C]verapamil (7.4 MBq/kg (0.2 mCi/kg; 0.12 μg/kg)) was administered to 12 healthy volunteers as an intravenous infusion over a period of approximately 1 min before and after at least a 1-h infusion of cyclosporin A (2.5 mg/kg/h; 2.8 ± 0.4 μmol/L) (16). Arterial blood samples and brain PET images were obtained at frequent intervals for 45 min. The brain accumulation of radioactivity (brain area under the curve/blood area under the curve) was determined in the presence and absence of cyclosporin A. The accumulation of radioactivity was increased by 88 ± 20% (1.02 ± 0.18 versus 0.55 ± 0.10; P < 0.001) in the presence of cyclosporin A without any apparent effect on cerebral blood flow, [¹¹C]verapamil metabolism, or plasma protein binding. These results indicate that P-gp activity at the human BBB can be measured.

Kortekaas et al. (17) reported that the DV (Logan analysis) of [¹¹C]verapamil in the midbrain was significantly increased (18%) in Parkinson’s disease (PD) patients (n = 5) compared with healthy controls (n = 5). The PD patients and controls had similar influx constants (K₁), suggesting reduced P-gp-mediated efflux in the PD patients. Four PD patients were taking antiparkinsonian medication, and none of the controls was on any medication. This suggests a decrease in P-pg activity at the BBB of PD patients.

[¹¹C]Verapamil PET is useful for objective monitoring of P-gp activity at the BBB [PubMed]. Internal dosimetry data for [¹¹C]verapamil in humans are not available in the literature.

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