Pharmacokinetics of 3'-fluoro-3'-deoxythymidine and 3'-deoxy-2',3'-didehydrothymidine in rats

Triphosphate Prodrugs of the Anti-HIV-Active Compound 3'-Deoxy-3'-fluorothymidine (FLT)

3'-Fluoro-3'-deoxythymidine (FLT) was identified as one of the most potent inhibitors of human immunodeficiency virus (HIV) replication. However, FLT also showed severe toxicity so that it was abundant as a potential chemotherapeutic agent. Here, we describe various triphosphate prodrugs of FLT aiming for (a) a bypass of all phosphorylation steps needed to convert the nucleoside analogue into its triphosphate (TP) form, (b) an intracellular delivery of hydrolytically and enzymatically stable triphosphate derivatives, and (c) increasing the selectivity for HIV-RT vs three cellular DNA polymerases including the mitochondrial DNA polymerase γ. γ-Alkylated FLTTP compounds fulfill all of these requirements because these compounds proved highly resistant to dephosphorylation and showed strong selectivity for HIV-RT. Moreover, a prodrug form of these compounds proved to be nontoxic in CEM cells.

Synthesis and Biological Evaluation of 5'-O-Fatty Acyl Ester Derivatives of 3'-Fluoro-2',3'-dideoxythymidine as Potential Anti-HIV Microbicides

A number of 5′-O-fatty acyl derivatives of 3′-fluoro-2′,3′-dideoxythymidine (FLT, 1) were synthesized. These conjugates were evaluated for their potential as topical microbicides with anti-HIV activity against cell-free (X4 and R5), cell-associated, and multidrug-resistant viruses. Compared to FLT and 3′-azido-2′,3′-dideoxythymidine (AZT), 5′-O-(12-azidododecanoyl) (5), 5′-O-myristoyl (6), and 5′-O-(12-thioethyldodecanoyl) (8) derivatives of FLT were found to be more active against both cell-free viruses (lymphocytotropic and monocytotropic strains) with EC50 values of 0.4 μM, 1.1 μM, and <0.2 μM, respectively, as well as cell-associated virus with EC50 values of 12.6, 6.4, and 2.3 μM, respectively. Conjugates 5, 6, and 8 exhibited >4 and >30 times better antiviral index than FLT and AZT, respectively. Conjugates 5 and 8 were significantly more potent than FLT against many multidrug-resistant strains. A comparison of the anti-HIV activity with the corresponding non-hydrolyzable ether conjugates suggested that ester hydrolysis to FLT and fatty acids is critical to enable anti-HIV activity. Cellular uptake studies were conducted using fluorescent derivatives of FLT attached with 5(6)-carboxyfluorescein through either β-alanine (23) or 12-aminododecanoic acid (24) spacers. The lipophilic fluorescent analog with a long chain (24) showed more than 12 times higher cellular uptake profile than the fluorescent analog with a short chain (23). These studies further confirmed that the attachment of fatty acids improved the cellular uptake of nucleoside conjugates. In addition, 5, 6, and 8 were the least cytotoxic and did not alter vaginal cell and sperm viability compared to the positive control, a commercial topical spermicide (N-9), which significantly decreased sperm and vaginal cell viability inducing the generation of proinflammatory cytokines.

Lymphatic Targeting of Anti-Human Immunodeficiency Virus Nucleosides: Pharmacokinetics of G′-Deoxy-2′,3′-Didehydrothymidine after Intravenous and Oral Administration of Dipalmitoylphosphatidyl Prodrug to Mice

The lymphatic system is a primary target for early anti-human immunodeficiency virus drug therapy. Strategies are currently being sought to enhance the delivery of nucleoside analogues such as 3′-deoxy-2′,3′-didehydrothymidine (stavudine; d4T) toward the lymph and lymph nodes. The purpose of this study was to synthesize dipalmitoylphosphatidyl-d4T (DPP-d4T) as a lipophilic prodrug of d4T and to evaluate the lymphatic distribution of d4T following administration of d4T and DPP-d4T to mice. The pharmacokinetics of d4T were characterized following administration of a single intravenous or oral dose of 50 mg kg−1 d4T and an equimolar dose (214 mg kg−1) of DPP-d4T. Concentrations of d4T in serum and lymph nodes were determined by HPLC. Following administration of d4T, the distribution of d4T into lymph nodes was rapid with maximum concentrations observed within 5 min after dosing. The AUC and half-life values of d4T in three groups of lymph nodes were similar to those in serum. Administration of DPP-d4T resulted in significantly lower concentrations of d4T in serum and lymph nodes. Approximately 67% of the intravenously administered DPP-d4T was biotransformed to parent compound. The apparent oral bioavailability of DPP-d4T was low. While the phospholipid prodrug did not increase d4T concentrations in the lymph nodes, it did provide an extended release of the parent nucleoside, resulting in sustained concentrations of d4T.

Pharmacokinetics of (-)-beta-D-dioxolane guanine and prodrug (-)-beta-D-2,6-diaminopurine dioxolane in rats and monkeys

(-)-beta-D-Dioxolane guanine (DXG) is a nucleoside analog possessing potent activity against human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2), and hepatitis B virus (HBV) in vitro. Owing to the limited aqueous solubility of DXG, (-)-beta-D-2,6-diaminopurine dioxolane (DAPD), a more water-soluble prodrug of DXG, is being developed for clinical use. The purpose of this study was to characterize the pharmacokinetics of DXG after administration of DXG and DAPD to rats and monkeys. After intravenous administration of DXG, plasma concentrations of the nucleoside declined in a biexponential manner, with a terminal-phase half-life of 0.44 +/- 0.14 hr (mean +/- SD) in rats and 2.3 hr in monkeys. Total clearance of DXG was 4.28 +/- 0.99 liters/hr/kg in rats and 0.72 liters/hr/kg in monkeys. Renal excretion of unchanged DXG accounted for approximately 50% of total clearance in both species. Steady state volume of distribution of DXG was 2.30 liters/kg in rats and 1.19 liters/kg in monkeys. After intravenous administration of DAPD to rats, prodrug concentrations declined with a half-life of 0.37 +/- 0.11 hr. DXG was rapidly generated from DAPD, with approximately 61% of the dose of DAPD being converted to DXG. After administration of DAPD to monkeys, only concentrations of metabolite DXG could be determined owing to rapid conversion of DAPD to DXG during sample collection. The half-lives of DAPD and DXG after intravenous administration determined from urinary excretion data were 0.8 +/- 0.4 and 1.6 +/- 0.2 hr, respectively. Oral bioavailability of DAPD was estimated to be approximately 30%.

Investigation of distribution, transport and uptake of anti-HIV drugs to the central nervous system

The distribution of currently available anti-HIV drugs into the CNS is reviewed with a focus on transport mechanisms. Among these drugs, nucleoside analogs are most well studied for their CNS distribution. The average reported values of the CSF/plasma steady-state concentration or corresponding AUC ratios are 0.23 (AZT), 0.06 (ddI), 0.04 (ddC), 0.49 (d4T), and 0.08 (3TC). Active efflux transport out of the CNS appears to be a predominant mechanism limiting nucleoside access to the CNS, although poor penetration may contribute to some extent for some polar nucleosides. The nature of the efflux pump for these drugs is speculated to be MRP-like transporter(s) in blood-brain and blood-CSF barriers. For non-nucleoside and protease inhibitors, much research remains to be done on the extent, time course, and mechanisms of their CNS distribution. The CNS penetration of some protease inhibitors is restricted by P-glycoprotein. A better understanding of transport mechanisms of anti-HIV drugs in the CNS is essential to develop approaches to enhance CNS delivery of available drugs and to identify new drugs less subject to active efflux transporter(s) in the CNS.

Development and optimization of anti-HIV nucleoside analogs and prodrugs: A review of their cellular pharmacology, structure-activity relationships and pharmacokinetics

Significant improvements in antiviral therapy have been realized over the past 10 years. Numerous nucleoside analogs, as well as prodrugs of active compounds, have been synthesized and tested for anti-HIV activity. In addition to the five nucleoside analogs currently used clinically for the treatment of HIV infection, a broad spectrum of anti-HIV nucleoside analogs (including 2',3'-dideoxynucleoside analogs, oxathiolanyl 2',3'-dideoxynucleoside analogs, dioxolanyl 2',3'-dideoxynucleoside analogs, carbocyclic 2',3'-dideoxynucleoside analogs and acyclic nucleoside analogs) and their prodrugs (including ester prodrugs, phospholipid prodrugs, dihydropyridine prodrugs, pronucleotides and dinucleotide analogs), targeted at HIV reverse transcriptase, are reviewed with focus on structure-activity relationships, cellular pharmacology and pharmacokinetics. Several of these anti-viral agents show promise in the treatment of AIDS.

Microdialysis studies of the distribution of stavudine into the central nervous system in the freely-moving rat

PURPOSE

To study the extent and time course of distribution of stavudine (d4T) into the central nervous system (CNS) and to investigate the transport mechanisms of antiviral nucleosides in the CNS.

METHODS

Microdialysis with on-line HPLC analysis was used to measure drug concentrations in the brain extracellular fluid (ECF) and cerebrospinal fluid (CSF) in the freely-moving rat. The in vivo recovery of d4T and zidovudine (AZT) was estimated by retrodialysis, which was validated by the zero-net flux method. The CNS distribution of d4T was investigated during iv and intracerebroventricular (icv) infusion. In the subsequent studies, the effect of AZT on CNS distribution of d4T was examined.

RESULTS

During iv infusion, d4T distributed rapidly into the CNS. Its brain ECF/plasma and CSF/plasma steady-state concentration ratios were 0.33 +/- 0.06 and 0.49 +/- 0.12, respectively (n = 15). During icv infusion, the steady-state d4T concentrations in the brain ECF were 23-fold higher than those during iv infusion, whereas its steady-state plasma levels were about the same for these two routes. Coadministration of AZT with d4T did not alter their respective brain distribution and systemic clearance at the concentrations examined. More importantly, the steady-state brain ECF/plasma and CSF/plasma concentration ratios of d4T were about 2-fold higher than those of AZT (0.15 +/- 0.04 and 0.25 +/- 0.08) determined in the same animals.

CONCLUSIONS

d4T readily crosses the blood-brain barrier (BBB) and blood-CSF barrier. An active efflux transport system in the BBB and blood-CSF barrier may be involved in transporting d4T out of the CNS. Direct icv administration of d4T can be used to enhance its brain delivery. Moreover, d4T exhibits a more favorable penetration into the CNS than AZT and therefore may be useful in the treatment of AIDS dementia complex.

Pharmacokinetics, oral bioavailability, and metabolic disposition in rats of (-)-cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl] cytosine, a nucleoside analog active against human immunodeficiency virus and hepatitis B virus

The pharmacokinetics and metabolism of the potent anti-human immunodeficiency virus and anti-hepatitis B virus compound, (-)-cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl] cytosine (FTC), were investigated in male CD rats. Plasma clearance of 10 mg of FTC per kg of body weight was biexponential in rats, with a half-life at alpha phase of 4.7 +/- 1.1 min (mean +/- standard deviation) and a half-life at beta phase of 44 +/- 8.8 min (n = 5). The total body clearance of FTC was 1.8 +/- 0.1 liters/h/kg, and the oral bioavailability was 90% +/- 8%. The volume of distribution at steady state (Vss) was 1.5 +/- 0.1 liters/kg. Increasing the dose to 100 mg/kg slowed clearance to 1.5 +/- 0.2 liters/kg/h, lowered the Vss to 1.2 +/- 0.2 liters/kg, and reduced the oral bioavailability to 65% +/- 15%. FTC in the brains of rats was initially less than 2% of the plasma concentration but increased to 6% by 2 h postdose. Probenecid elevated levels of FTC in plasma as well as in brains but did not alter the brain-to-plasma ratio. The urinary and fecal recoveries of unchanged FTC after a 10-mg/kg intravenous dose were 87% +/- 3% and 5% +/- 1.6%, respectively. After a 10-mg/kg oral dose, respective urinary and fecal recoveries were 70% +/- 2.5% and 25% +/- 1.6%. Two sulfoxides of FTC were observed in the urine, accounting for 0.4% +/- 0.03% and 2.7% +/- 0.2% of the intravenous dose and 0.4% +/- 0.06% and 2.5% +/- 0.3% of the oral dose. Also observed were 5-fluorocytosine, representing 0.4% +/- 0.06% of the intravenous dose and 0.4% +/- 0.07% of the oral dose, and FTC glucuronide, representing 0.7% +/- 0.2% of the oral dose and 0.4% +/- 0.2% of the intravenous dose. Neither deaminated FTC nor 5-fluorouracil was observed in the urine (less than 0.2% of dose). The high oral availability and minimal metabolism of FTC encourage its further preclinical development.

Pharmacokinetics of the anti-human immunodeficiency virus nucleoside analog stavudine in cynomolgus monkeys

Stavudine was administered (15 mg/kg of body weight) intravenously and orally to two monkeys in a randomized crossover study. Plasma and urine samples were analyzed for stavudine by high-performance liquid chromatography, and pharmacokinetic parameters were derived by a noncompartmental method. Total body clearance of stavudine was 0.64 liters/h/kg, with a steady-state volume of distribution of 0.68 liters/kg, a terminal half-life of 0.83 h, a urinary recovery of 44%, and an oral bioavailability of 80%. These values were reasonably similar to those reported for patients with AIDS or AIDS-related complex.

Determination of stavudine, a new antiretroviral agent, in human plasma by reversed-phase high-performance liquid chromatography with ultraviolet detection

A sensitive high-performance liquid chromatographic assay has been developed to determine the levels of a new antiretroviral agent, stavudine (2',3'-didehydro-3'-deoxythymidine, d4T), in human plasma. Didanosine (2',3'-dideoxyinosine, ddI) was used as the internal standard. The very selective sample pretreatment involved solid-phase extraction using silica gel columns. Chromatography was carried out on a mu Bondapak phenyl column, using a mobile phase of 0.005 M phosphate buffer (pH 6.8)-methanol (90:10, v/v) and ultraviolet detection at 265 nm. The method has been validated, and stability tests under various conditions have been performed. The detection limit is 10 ng/ml (using 500-microliters human plasma samples). The bioanalytical assay has been used in a single pharmacokinetic experiment in a rat to investigate the applicability of the method in vivo.

Three-drug synergistic inhibition of HIV-1 replication in vitro by 3'-fluoro-3'-deoxythymidine, recombinant soluble CD4, and recombinant interferon-alpha

3'-Fluoro-3'deoxythymidine (FLT), recombinant soluble CD4 (CD4), and recombinant interferon-alpha (IFN alpha) were evaluated in two- and three-drug regimens against HIV-1 replication in vitro. Peripheral blood mononuclear cells were studied using p24 antigen production as the virologic endpoints. FLT showed 2.5-fold higher efficacy and a similar selectivity index to zidovudine. Drug interactions were evaluated by the median effect principle and the isobologram technique. FLT, CD4, and interferon alpha at noncytotoxic concentrations inhibited HIV-1 synergistically in two- and three-drug combinations with a combination index smaller than one and dose reduction index greater than one. The three-drug regimen provided greater virus suppression than the two-drug regimen. These results suggest that FLT is an alternative agent to AZT for the treatment of HIV infection either as a single agent or in combination with CD4 and/or interferon-alpha.

Comparisons of anti-human immunodeficiency virus activities, cellular transport, and plasma and intracellular pharmacokinetics of 3'-fluoro-3'-deoxythymidine and 3'-azido-3'-deoxythymidine

3'-Fluoro-3'-deoxythymidine (FLT), a candidate anti-AIDS compound in clinical trials, showed anti-human immunodeficiency virus type 1 (HIV-1) potency (50% effective concentration, 0.0052 microM) slightly better than or equal to that of 3'-azido-3'-deoxythymidine (AZT) in MT4 cells and was threefold more potent in H9 cells. There was no FLT resistance demonstrable in the AZT-resistant HIV-1 strains. Both FLT and AZT showed low cytotoxicity for MT4 cells, with selectivity indices (efficacy/toxicity ratio) of greater than 47,000 and greater than 33,000, respectively. Cellular permeation of FLT and thymidine (dThd) was greater than that of AZT, and FLT and dThd permeated the cell membranes by a carrier-mediated mechanism as well as by simple diffusion, as indicated by the existence of nitrobenzylthioinosine-5'-monophosphate-sensitive and -insensitive components. By contrast, transport of AZT into cells was by simple diffusion. The intracellular level of the triphosphate of FLT (FLTTP) in MT4 cells was two- to threefold higher than that of AZT (AZTTP) after exposure to 1.8 microM each compound for 12 h. The elimination kinetics of FLTTP and AZTTP in HIV-1-infected MT4 cells in fresh medium showed biphasic patterns, with initial half-lives of 1.03 and 1.09 h, respectively. In phytohemagglutinin-stimulated human peripheral blood lymphocytes, the FLTTP level was increased 59-fold compared with that in unstimulated cells at 12 h, was four- to sixfold higher than the level of AZTTP in stimulated cells at 12 h, and remained four- to fivefold higher during a 4-h elimination period in fresh medium and twofold higher at the end of a 12-h elimination period. Two- to eightfold more [3H]AZT than [3H]FLT was incorporated into the host cell DNA, and both [3H]AZT and [3H]FLT remained persistently incorporated for over 24 h. The incorporated [3H]AZT and [3H]FLT were alkali labile, whereas incorporated [3H]dThd was alkali stable. Pharmacokinetics of FLT in plasma of monkeys after intravenous (i.v.) administration showed that the FLT concentration in plasma declined, with a half-life of 1.19 +/- 0.1 h; the steady-state volume of distribution was 0.93 +/- 0.2 liter/kg of body weight, and total clearance was 0.56 +/- 0.15 liter/kg. Oral bioavailability of FLT was excellent and comparable to i.v. bioavailability in terms of areas under the concentration-time curves for three monkeys. Of the total dose, 41 to 61% was excreted in urine as unchanged FLT, and only 3.2 to 7.4% of the total dose was identified as glucuronide-conjugated FLT in urine 48 h after i.v. administration to monkeys. We conclude that FLT exhibits an anti-HIV-1 potency similar to that of AZT but with slightly better selectivity of effects and with higher intracellular active metabolite levels.