Reversal of the cytotoxicity of 3'-amino-3'-deoxythymidine by pyrimidine deoxyribonucleosides

Transport, metabolism and elimination mechanisms of anti-HIV agents

Currently available anti-HIV drugs can be classified into three categories: nucleoside analogue reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, and protease inhibitors. Knowledge of these anti-HIV drugs in various physiological or pharmacokinetic compartments is essential for design and development of drug delivery systems for the treatment of HIV infection. The input and output of anti-HIV drugs in the biological systems are described by their transport and metabolism/elimination in this review. Transport mechanisms of anti-HIV agents across various biological barriers, i.e., gastrointestinal wall, skin, mucosa, blood cerebrospinal barrier, blood-brain barrier, placenta, and cellular membranes, are discussed. Their fates during and after systemic absorption and their metabolism-related drug interactions are reviewed. Many anti-HIV drugs presently marketed in the US bear some significant drawbacks such as relatively short half-life, low bioavailability, poor penetration into the central nervous system, and undesirable side effects. Efforts have been made to design drug delivery systems for the anti-HIV agents to: (1) reduce the dosing frequency; (2) increase the bioavailability and decrease the degradation/metabolism in the gastrointestinal tract; (3) improve the CNS penetration and inhibit the CNS efflux; and (4) deliver them to target cells selectively with minimal side effects. We hope to stimulate further interests in the area of controlled delivery of anti-HIV agents by providing current status of transport and metabolism/elimination of these agents.

Role of human liver P450s and cytochrome b5 in the reductive metabolism of 3'-azido-3'-deoxythymidine (AZT) to 3'-amino-3'-deoxythymidine

Our laboratory has shown that human liver microsomes metabolize the anti-HIV drug 3'-azido-3'-deoxythymidine (AZT) via a P450-type reductive reaction to a toxic metabolite 3'-amino-3'-deoxythymidine (AMT). In the present study, we examined the role of specific human P450s and other microsomal enzymes in AZT reduction. Under anaerobic conditions in the presence of NADPH, human liver microsomes converted AZT to AMT with kinetics indicative of two enzymatic components, one with a low Km (58-74 microM) and Vmax (107-142 pmol AMT formed/min/mg protein) and the other with a high Km (4.33-5.88 mM) and Vmax (1804-2607 pmol AMT formed/min/mg). Involvement of a specific P450 enzyme in AZT reduction was not detected by using human P450 substrates and inhibitors. Antibodies to human CYP2E1, CYP3A4, CYP2C8, CYP2C9, CYP2C19, and CYP2A6 were also without effect on this reaction. NADH was as effective as NADPH in promoting microsomal AZT reduction, raising the possibility of cytochrome b5 (b5) involvement. Indeed, AZT reduction among six human liver samples correlated strongly with microsomal b5 content (r2 = 0.96) as well as with aggregate P450 content (r2 = 0.97). Upon reconstitution, human liver b5 plus NADH:b5 reductase and CYP2C9 plus NADPH:P450 reductase were both effective catalysts of AZT reduction, which was also supported when CYP2A6 or CYP2E1 was substituted for CYP2C9. Kinetic analysis revealed an AZT Km of 54 microM and Vmax of 301 pmol/min for b5 plus NADH:b5 reductase and an AZT Km of 103 microM and Vmax of 397 pmol/min for CYP2C9 plus NADPH:P450 reductase. Our results indicate that AZT reduction to AMT by human liver microsomes involves both b5 and P450 enzymes plus their corresponding reductases. The capacity of these proteins and b5 to reduce AZT may be a function of their heme prothestic groups.

Molecular basis for thymidine modulation of the efficacy and toxicity of alkylating agents

The antitumor activity of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in mice previously was shown to be markedly enhanced by co-administration of thymidine. We have examined the cellular mechanisms underlying the augmentation effect of thymidine. It was found that thymidine did not increase the cytotoxicity of BCNU for B16/F10 melanoma or L1210 leukemia cells in vitro. Instead, thymidine appeared to augment the activity of tumor-specific cytotoxic T-cells in tumor-bearing mice, which specifically rejected a secondary challenge with the B16/F10 tumor. Thus, development of an antitumor immune response is facilitated by thymidine in BCNU-induced immunosuppressed mice. These preclinical studies suggested that combination therapy with alkylating agents and thymidine may be a more efficacious and less toxic anticancer therapy. The potential efficacy of the sequential administration of dacarbazine (DTIC), BCNU, and thymidine in patients with advanced malignant melanoma was investigated. As predicted from animal studies, sequential administration of DTIC, BCNU, and thymidine is a relatively nontoxic therapy for metastatic melanoma. This treatment induced durable responses in up to 35% of patients, and hence is superior to many commonly used toxic combination chemotherapies. The mechanism of action, although not well characterized, is thought to be mediated through protection of the cellular immune process, as well as organ function, from alkylating agent toxicity through modulation of DNA repair enzymes such as O(6)-alkylguanine-DNA alkyltransferase in normal tissue. Thus, thymidine is a biomodulator, which not only protects patients from hematologic, pulmonary, and hepatic toxicities associated with DTIC and BCNU chemotherapy, but also potentiates therapeutic efficacy.

Comparative pharmacokinetics of zidovudine and its toxic catabolite 3'-amino-3'-deoxythymidine in HIV-infected patients

The plasma pharmacokinetics of zidovudine (ZDV, 3'-azido-3'-deoxythymidine) and its toxic catabolite 3'-amino-3'-deoxythymidine (AMT) was investigated in six HIV-infected patients receiving 100 or 500 mg of ZDV by oral administration. Zidovudine plasma pharmacokinetic parameters were in good agreement with previously reported data with a total plasma clearance (Cl) of 2.33 and 2.49 liters/hr/kg and an apparent elimination t1/2 of 1.14 and 1.20 hr at 100- and 500-mg doses, respectively. 3'-Amino-3'-deoxythymidine was detectable in the plasma of all patients, with maximum plasma levels (Cmax) being reached within 2 hr post-dosing. No dose relationship in the formation of AMT expressed as the area under the plasma level-time curve (AUC) was established within the studied dose range of ZDV. The AUCAMT was 145.4 and 92.4 ng/ml.hr after administration of 100 and 500 mg of ZDV, respectively. These results reflect an unexpectedly large interindividual variation in the formation of AMT. The lack of linearity in AMT pharmacokinetics prevents the prediction of its plasma levels based only on the administered oral dose of ZDV.

Determination of 3'-amino-3'-deoxythymidine, a cytotoxic metabolite of 3'-azido-3'-deoxythymidine, in human plasma by ion-pair high-performance liquid chromatography

A sensitive high-performance liquid chromatographic assay has been developed to determine the levels of 3'-amino-3'-deoxythymidine (AMT), a cytotoxic metabolite of 3'-azido-3'-deoxy-thymidine (AZT, zidovudine), in human plasma. The sample pretreatment involved solid-phase extraction using cation-exchange extraction columns. Chromatography was carried out on a C8 column, using a mobile phase of methanol-0.01 M ammonium acetate (pH 5)-0.25 M sodium dioctylsulfosuccinate (60:40:4, v/v/v) and ultraviolet detection at 265 nm. The method has been validated, and stability tests under various conditions have been performed. The lower limit of quantitation is 5 ng/ml (using 500-microliters human plasma samples). The bioanalytical assay has been used for the determination of AMT in patients with AIDS who used AZT.

Ribo- and deoxyribonucleoside effect on 3'-amino-2',3'-dideoxycytidine-induced cytotoxicity in cultured L1210 cells

3'Amino-2',3'-dideoxycytidine (3'-NH2-dCyd) is a potent inhibitor of the replication of cultured L1210 cells, with an IC50 of 1 microM. When ribo- and deoxyribonucleosides were examined for their effects on 3'-NH2-dCyd-induced cytotoxicity, only dCyd could both prevent and reverse these effects. Furthermore, even when the maximum increase in modal cell volume was allowed to develop (24 hr) in the presence of 2.5 microM 3'-NH2-dCyd, the addition of 25 microM dCyd to the medium containing 3'-NH2-dCyd reduced the modal cell volume nearly to control levels within 24 hr. Examination of the viability of these cells by colony formation in soft agar, following as much as a 9.5-hr exposure of 1, 2.5 and 10 microM 3'-NH2-dCyd, revealed that the lethal effects of the 3'-NH2-dCyd treatment were not observed only when 25 microM dCyd was added to the medium during this time. However, the lethality of a 24-hr exposure of 2.5 and 10 microM 3'-NH2-dCyd could not be prevented either by removal of the drug from the medium or by a 24-hr exposure of the medium containing 3'-NH2-dCyd to 25 microM dCyd. When the effect of 3'-NH2-dCyd on DNA biosynthesis in L1210 cells was examined, it was found that radiolabeled dAdo incorporation decreased by approximately 60, 80 or 90% following a 2.5-hr exposure to 2.5, 10 or 20 microM 3'-NH2-dCyd respectively. The addition of 25 microM dCyd under the same conditions resulted in a greater amount of dAdo incorporation compared to the unrescued cultures. Deamination of 3'-NH2-dCyd by partially purified human cytidine-deoxycytidine deaminase was about 2.5% that of either Cyd or dCyd deamination. The deaminated derivative, 3'-amino-2',3'-dideoxyuridine, was significantly less cytotoxic even at 50 microM.