N. Harry Hendrikse

Multidrug resistance associated proteins (MRPs) and P-glycoprotein (P-gp) are involved in hepatobiliary transport of various compounds. Our aim was (1) to define transporter specificity of the cholescintigraphic agents ^99mTc-HIDA and ^99mTc-MIBI, which are used clinically for myocardial perfusion measurements; and (2) to deduce MRP and P-gp functions in vivo from hepatic ^99mTc kinetics. Accumulation of radioactivity was measured in the human tumor cell lines GLC₄, GLC₄/ADR_150x (MRP1-overexpressing/P-gp-negative) and GLC₄/P-gp (P-gp-overexpressing). Bile secretion was quantified in untreated and in glutathione-depleted control and MRP2-deficient (GY/TR⁻) rats. Hepatobiliary transport was measured using a gamma camera in both types of rats. ^99mTc-HIDA accumulated 5.8-fold less in GLC₄/ADR_150x calls than in GLC₄ or GLC₄/P-gp cells. In GLC₄/ADR_150x, the cellular ^99mTc-HIDA content was increased 3.4-fold by the MRP1,2 inhibitor MK571 (50 μM), while MK571 had no measurable effect in GLC₄ and GLC₄/P-gp cells. ^99mTc-MIBI accumulated less in GLC₄/P-gp and GLC₄/ADR_150x cells than in GLC₄ cells. Bile secretion of ^99mTc-HIDA was impaired in GY/TR⁻ compared to control rats and not affected by glutathione depletion in GY/TR⁻ rats. Hepatic secretion of ^99mTc-HIDA was slower in GY/TR⁻ (t_1/2 40 min) than in control rats (t_1/2 7 min). Bile secretion of ^99mTc-MIBI was similar in both rat strains and impaired by glutathione depletion in control rats only, indicating compensatory activity of additional transporter(s) in GY/TR⁻ rats. ^99mTc-HIDA is transported only by MRP1,2 only, while ^99mTc-MIBI is transported by P-gp and MRP1,2. The results indicate that hepatic P-gp and MRP1,2 function can be assessed in vivo by sequential use of both radiopharmaceuticals.

Objective: To evaluate [¹¹C]verapamil kinetics in humans. Methods: After intravenous injection of [¹¹C]verapamil (370 MBq, specific activity >1600 GBq/mmol), kinetics were evaluated in five cancer patients using positron emission tomography (PET). Results: One hour after injection, accumulation of [¹¹C] in lungs, heart and tumour was 43.0%, 1.3% and 0.9% of the injected verapamil dose, respectively. Half-lives of [¹¹C]verapamil in these tissues were 46.2 min, 73.8 min and 23.7 min, respectively. Conclusions: Intravenously administered [¹¹C] was mainly extracted by the lungs. Transport of a [¹¹C]verapamil bolus out of solid tumour tissue is relatively fast.

Abstract: Chemotherapeutic treatment of cancer patients is often unsuccessful, due to the involvement of various mechanisms, leading to multidrug resistance (MDR). In this review, I describe the mechanisms involved in MDR. Furthermore, results obtained by imaging of P-glycoprotein (P-gp) and the multidrug resistance associated protein (MRP) are reviewed. Single photon emission computed tomography (SPECT) and positron emission tomography (PET) are unique techniques to study P-gp- and MRP-mediated transport. The radiopharmaceutical 99mTc-sestamibi is a substrate for both P-gp and MRP. This tracer has been used for tumor imaging in clinical studies, and to visualize blockade of P-gp mediated transport after modulation of the P-gp pump. Other 99mTcradiopharmaceuticals such as 99mTc- tetrofosmin and several 99mTc-Q-complexes are also substrates for Pgp. Until now, for these compounds only results from in vitro and animal studies are available. For quantification of P-gp mediated transport with PET in vivo, several agents, such as [11C]colchicine, [11C]verapamil and [11C]daunorubicin have been evaluated. In vivo results suggest that these radiopharmaceuticals can be used to image P-gp function in tumors. 124I and 76Br radiolabeled doxorubicin analogues are also useful to examine P-gp mediated transport. Leukotrienes are specific substrates for MRP. Therefore, N-[11C]acetyl-leukotriene E4 provides the opportunity to study MRP function non-invasively. Results obtained with this radiopharmaceutical in MRP2 mutated GY/TR- rats indicate visualization of MRP-mediated transport. This tracer enables to study MRP transport function abnormalities in vivo such as in Dubin-Johnson patients, who are MRP2 gene deficient. In conclusion, it is feasible to study the functionality of MDR transporters in vivo, both with SPECT and with PET. Such imaging techniques may become an important factor in the development of novel chemotherapeutic drugs.

One of the mechanisms for multidrug resistance (MDR) of tumors is an overexpression of the P-glycoprotein (P-gp). The cytostatic agent daunorubicin and the modulator verapamil were labeled with 11C to probe P-gp with PET. Carbon-11-daunorubicin was prepared from 11CCH2N2 with an aldehyde precursor, followed by hydrolysis. Carbon-11-verapamil was synthesized by 11C-methylation. Both tracers were evaluated by investigating pharmacokinetics in rats and in vitro cell kinetics using human ovarian carcinoma cells. Amounts of 111 MBq 11C-daunorubicin were prepared. Biodistribution studies of 11C-daunorubicin in male Wistar rats showed dose-dependent pharmacokinetics, whereas with 11C-verapamil the pharmacokinetics were dose independent. In in vitro experiments with cells, the ratio of accumulation of 11C-daunorubicin in drug sensitive/resistant cell lines was 16. Addition of verapamil resulted in increased accumulation of 11C-daunorubicin in the resistant cell line. The ratios of 11C-verapamil accumulation in drug-sensitive versus the MDR counterpart were 4-5. Carbon-11-daunorubicin and 11C-verapamil both have potential for in vivo probing of P-glycoprotein with PET.