Whole blood or plasma: what is the ideal matrix for pharmacokinetic-driven drug candidate selection?

In the present era of drug development, quantification of drug concentrations following pharmacokinetic studies has preferentially been performed using plasma as a matrix rather than whole blood. However, it is critical to realize the difference between measuring drug concentrations in blood versus plasma and the consequences thereof. Pharmacokinetics using plasma data may be misleading if concentrations differ between plasma and red blood cells (RBCs) because of differential binding in blood. In this review, factors modulating the partitioning of drugs into RBCs are discussed and the importance of determining RBC uptake of drugs for drug candidate selection is explored. In summary, the choice of matrix (plasma vs whole blood) is an important consideration to be factored in during drug discovery.

18F-FAZA PET imaging response tracks the reoxygenation of tumors in mice upon treatment with the mitochondrial complex I inhibitor BAY 87-2243.

PURPOSE: We describe a noninvasive PET imaging method that monitors early therapeutic efficacy of BAY 87-2243, a novel small-molecule inhibitor of mitochondrial complex I as a function of hypoxia-inducible factor-1α (HIF1α) activity. EXPERIMENTAL DESIGN: Four PET tracers [(18)F-FDG, (18)F-Fpp(RGD)2, (18)F-FLT, and (18)F-FAZA] were assessed for uptake into tumor xenografts of drug-responsive (H460, PC3) or drug-resistant (786-0) carcinoma cells. Mice were treated with BAY 87-2243 or vehicle. At each point, RNA from treated and vehicle H460 tumor xenografts (n = 3 each) was isolated and analyzed for target genes. RESULTS: Significant changes in uptake of (18)F-FAZA, (18)F-FLT, and (18)F-Fpp(RGD)2 (P < 0.01) occurred with BAY 87-2243 treatment with (18)F-FAZA being the most prominent. (18)F-FDG uptake was unaffected. (18)F-FAZA tumor uptake declined by 55% to 70% (1.21% ± 0.10%ID/g to 0.35 ± 0.1%ID/g; n = 6, vehicle vs. treatment) in both H460 (P < 0.001) and PC3 (P < 0.05) xenografts 1 to 3 days after drug administration. (18)F-FAZA uptake in 786-0 xenografts was unaffected. Decline occurred before significant differences in tumor volume, thus suggesting (18)F-FAZA decrease reflected early changes in tumor metabolism. BAY 87-2243 reduced expression of hypoxia-regulated genes CA IX, ANGPTL4, and EGLN-3 by 99%, 93%, and 83%, respectively (P < 0.001 for all), which corresponds with reduced (18)F-FAZA uptake upon drug treatment. Heterogeneous expression of genes associated with glucose metabolism, vessel density, and proliferation was observed. CONCLUSIONS: Our studies suggest suitability of (18)F-FAZA-PET as an early pharmacodynamic monitor on the efficacy of anticancer agents that target the mitochondrial complex I and intratumor oxygen levels (e.g., BAY 87-2243).

Molecular PET and PET/CT imaging of tumour cell proliferation using F-18 fluoro-L-thymidine: a comprehensive evaluation.

Positron emission tomography (PET) using F-18 fluoro-3'-deoxy-3-L-fluorothymidine (FLT) offers noninvasive assessment of cell proliferation in vivo. The most important application refers to the evaluation of tumour proliferative activity, representing a key feature of malignancy. Most data to date suggest that FLT is not a suitable biomarker for staging of cancers. This is because of the rather low fraction of tumour cells that undergo replication at a given time with subsequently relatively low tumour FLT uptake. In addition, generally, the high FLT uptake in liver and bone marrow limits the diagnostic use. We describe the current status on preclinical and clinical applications of FLT-PET including our own experience in brain tumours. The future of FLT-PET probably lies in the evaluation of tumour response to therapy and more importantly, in the prediction of early response in the course of treatment. The level of FLT accumulation in tumours depends on thymidine kinase 1 activity and on the therapy-induced activation of the salvage pathway and expression of nucleoside transporters. Therefore, cytostatic agents that cause arrest of the cell cycle in the S-phase may initially increase FLT uptake rather than reducing the tumour cell accumulation. In addition, agents that block the endogenous thymidine pathway may lead to overactivity of the salvage pathway and increase tumour FLT uptake. In contrast, many therapeutic agents inhibit both pathways and subsequently reduce tumour FLT uptake. Further studies comparing FLT with F-18 fluorodeoxyglucose-PET will be important to determine the complementary advantage of FLT-PET in early cancer therapy response assessment. Further research should be facilitated by simplified synthesis of FLT with improved yields and an increasing commercial availability.

Synthesis and biodistribution of 3'-fluoro-5-[(131)I]iodo-2'-deoxyuridine: a comparative study of [(131)I]FLIdU and [(18)F]FLT.

The radioiodinated 3'-fluorothymidine (FLT) analogue 3'-fluoro-5-[(131)I]iodo-2'-deoxyuridine ([(131)I]FLIdU) was synthesized, with iodine mimicking the methyl group of pyrimidine. [(131)I]FLIdU was accessible by direct electrophilic iodination using Iodogen as oxidant. Optimized amounts of the oxidant allowed radiochemical yields of about 70% after a reaction time of 10 min in an aqueous buffer medium at 90 degrees C. The uptake of [(131)I]FLIdU in a DoHH2 leukemia xenograft mouse model and in healthy mice revealed moderate FLIdU accumulation, followed by a significant washout of activity in proliferating tissues such as splenic and tumor tissues. In contrast, intraperitoneal coinjection with [(18)F]FLT showed high uptake and high activity retention up to 2 h, in both splenic and tumor tissues. Uptake in stomach tissues and increasing fractions of [(131)I]iodide in urine indicated metabolic instability of [(131)I]FLIdU due to rapid deiodination. Therefore, [(131)I]FLIdU alone does not seem to be a promising compound, neither for diagnostic imaging nor for potential therapeutic applications.

Antiviral effects of 3'-fluorothymidine and 3'-azidothymidine in cynomolgus monkeys infected with simian immunodeficiency virus.

An acute infection with simian immunodeficiency virus (SIVSM) in cynomolgus monkeys was used to evaluate the antiviral effects of 3'-fluorothymidine (FLT) and 3'-azidothymidine [zidovudine (ZDV)]. Neither compound prevented the infection despite dosing prior to virus inoculation. FLT was about ten times more potent than ZDV in delaying the appearance of SIVSM antigen in the monkeys. The serum half-life of FLT was longer than that of ZDV and ZDV was bound to plasma proteins to about 60% while FLT was virtually unbound. It is proposed that the in vivo difference in potency between ZDV and FLT could, at least partly, be explained as the combined effects of a longer plasma half-life and a higher free concentration of FLT and possibly a higher intracellular concentration of the triphosphate of FLT.