Clinical pharmacokinetics of stavudine

The Application of Prodrugs as a Tool to Enhance the Properties of Nucleoside Reverse Transcriptase Inhibitors

Antiretroviral Therapy (ART) is an effective treatment for human immunodeficiency virus (HIV) which has transformed the highly lethal disease, acquired immunodeficiency syndrome (AIDS), into a chronic and manageable condition. However, better methods need to be developed for enhancing patient access and adherence to therapy and for improving treatment in the long term to reduce adverse effects. From the perspective of drug discovery, one promising strategy is the development of anti-HIV prodrugs. This approach aims to enhance the efficacy and safety of treatment, promoting the development of more appropriate and convenient systems for patients. In this review, we discussed the use of the prodrug approach for HIV antiviral agents and emphasized nucleoside reverse transcriptase inhibitors. We comprehensively described various strategies that are used to enhance factors such as water solubility, bioavailability, pharmacokinetic parameters, permeability across biological membranes, chemical stability, drug delivery to specific sites/organs, and tolerability. These strategies might help researchers conduct better studies in this field. We also reported successful examples from the primary therapeutic classes while discussing the advantages and limitations. In this review, we highlighted the key trends in the application of the prodrug approach for treating HIV/AIDS.

Chemoenzymatic synthesis of bridged homolyxofuranosyl pyrimidine nucleosides: Bicyclic AZT analogues

A greener chemo-enzymatic methodology has been developed for the synthesis of conformationally restricted diastereomeric homolyxofuranosyl pyrimidines (AZT analogue), i.e., (5'R)-3'-azido-3'-deoxy-2'-O,5'-C-bridged-β-d-homolyxofuranosyl-uracil and thymine starting from inexpensive diacetone-d-glucofuranose in 18% and 21% overall yields, respectively. In one of the key steps in multistep synthesis of bicyclic AZT analogues, the primary hydroxyl group of 3'-azido-3'-deoxy-β-d-glucofuranosyl pyrimidines has been acetylated using Novozyme® 435 in THF in 92% and 97% yields, respectively. The monoacetylated nucleoside was converted to desired bicyclic AZT analogue in two steps in an overall yield of 82% and 83%, respectively.

Chemical and chemoenzymatic routes to bridged homoarabinofuranosylpyrimidines: Bicyclic AZT analogues

Conformationally restricted diastereomeric homoarabinofuranosylpyrimidines (AZT analogue), i.e., (5′R)-3′-azido-3′-deoxy-2′-O,5′-C-bridged-β-ᴅ-homoarabinofuranosylthymine and -uracil had been synthesized starting from diacetone ᴅ-glucofuranose following chemoenzymatic and chemical routes in 34–35% and 24–25% overall yields, respectively. The quantitative and diastereoselective acetylation of primary hydroxy over two secondary hydroxy groups present in the key nucleoside precursor was mediated with Lipozyme^® TL IM in 2-methyltetrahydrofuran following a chemoenzymatic pathway. Whereas, the protection of the primary hydroxy over the lone secondary hydroxy group in the key azido sugar precursor was achieved using bulky tert-butyldiphenylsilyl chloride (TBDPS-Cl) in pyridine in 92% yield following a chemical synthetic pathway. The chemoenzymatic method was found to be superior over the chemical method in respect of the number of synthetic steps and overall yield of the final product.

Geminal Diheteroatomic Motifs: Some Applications of Acetals, Ketals, and Their Sulfur and Nitrogen Homologues in Medicinal Chemistry and Drug Design

Acetals and ketals and their nitrogen and sulfur homologues are often considered to be unconventional and potentially problematic scaffolding elements or pharmacophores for the design of orally bioavailable drugs. This opinion is largely a function of the perception that such motifs might be chemically unstable under the acidic conditions of the stomach and upper gastrointestinal tract. However, even simple acetals and ketals, including acyclic molecules, can be sufficiently robust under acidic conditions to be fashioned into orally bioavailable drugs, and these structural elements are embedded in many effective therapeutic agents. The chemical stability of molecules incorporating geminal diheteroatomic motifs can be modulated by physicochemical design principles that include the judicious deployment of proximal electron-withdrawing substituents and conformational restriction. In this Perspective, we exemplify geminal diheteroatomic motifs that have been utilized in the discovery of orally bioavailable drugs or drug candidates against the backdrop of understanding their potential for chemical lability.

Can We Improve Stavudine's Safety Profile in Children? Pharmacokinetics of Intracellular Stavudine Triphosphate with Reduced Dosing

Stavudine remains a useful replacement option for treatment for HIV⁺ children. WHO reduced the adult dose to 30 mg twice daily, which maintains efficacy and lowers mitochondrial toxicity. We explored intracellular stavudine triphosphate levels in children receiving a reduced dose of 0.5 to 0.75 mg/kg of body weight twice daily to investigate whether a similar dose optimization can safely be made. A population pharmacokinetic model was developed to describe the pharmacokinetics of intracellular stavudine triphosphate in 23 HIV⁺ children and 24 HIV⁺ adults who received stavudine at 0.5 mg/kg and 20 mg twice daily for 7 days, respectively. Simulations were employed to optimize the pediatric dosing regimen to match exposures in adults receiving the current WHO-recommended dose of 30 mg twice daily. A biphasic disposition model with first-order appearance and disappearance described the pharmacokinetics of stavudine triphosphate. The use of allometric scaling with fat-free mass characterized well the pharmacokinetics in both adults and children, and no other significant effect could be detected. Simulations of 30 mg twice daily in adults predicted median (interquartile range [IQR]) stavudine triphosphate minimum drug concentration (C _min) and maximum drug concentration (C _max) values of 13 (10 to 19) and 45 (38 to 53) fmol/10⁶ cells, respectively. Targeting this exposure, simulations in HIV⁺ children were used to identify a suitable weight-band dosing approach (0.5 to 0.75 mg/kg), which was predicted to achieve median (IQR) C _min and C _max values of 13 (9 to 18) and 49 (40 to 58) fmol/10⁶ cells, respectively. Weight-band dosing using a stavudine dose of 0.5 to 0.75 mg/kg is proposed, and it shows comparable exposures to adults receiving the current WHO-recommended dose of 30 mg twice daily. Our pharmacokinetic results suggest that the decreased stavudine dose in children >2 years would have a reduced toxic effect while retaining antiretroviral efficacy.

Association of Nevirapine Levels with Rash or Hepatotoxicity Among HIV-Infected Thai Women

BACKGROUND

We performed a nested case-control study of Thai women prescribed nevirapine-based antiretroviral therapy (ART) to determine if development of rash or hepatotoxicity during the first 24 weeks of treatment is associated with plasma nevirapine concentrations.

METHOD

From May 2005-January 2007, we enrolled 217 women initiating nevirapine-based ART in Thailand. Cases (n = 54) were women who during the first 24 weeks of treatment with nevirapine developed rash (any grade, n = 42) or hepatotoxicity (≥grade 2, n = 22, [10 had both]). Controls were the next enrolled woman who was confirmed not to meet the case definition during the first 24 weeks. Nevirapine concentrations after the two week lead-in dose of 200 mg once daily were compared between cases and controls by Wilcoxon rank-sum tests.

RESULTS

We found no difference in Week 2 pre-dose nevirapine concentrations: cases median = 3,528 ng/mL (n = 24), controls median = 3,150ng/mL (n = 30), p = 0.5. Cases had higher post-dose nevirapine concentrations (median = 6,150 ng/mL, n = 21) than controls (median = 4,746 ng/mL, n = 20, p = 0.02). When limited to cases who developed a rash at Week 2, we found no differences in the pre-dose (median = 3,270 ng/mL, n = 12, p = 0.9) or post-dose nevirapine concentration (median = 5,443 ng/mL, n = 9, p = 0.4) compared with controls.

CONCLUSIONS

We cannot conclude definitively that nevirapine concentrations at two weeks of therapy are associated with rash or hepatotoxicity. It is unlikely that therapeutic drug monitoring at that time will improve identification of patients at risk for rash or hepatotoxicity.

Computational models to assign biopharmaceutics drug disposition classification from molecular structure

PURPOSE

We applied in silico methods to automatically classify drugs according to the Biopharmaceutics Drug Disposition Classification System (BDDCS).

MATERIALS AND METHODS

Models were developed using machine learning methods including recursive partitioning (RP), random forest (RF) and support vector machine (SVM) algorithms with ChemDraw, clogP, polar surface area, VolSurf and MolConnZ descriptors. The dataset consisted of 165 training and 56 test set molecules.

RESULTS

RF model 3, RP model 1, and SVM model 1 can correctly predict 73.1, 63.6 and 78.6% test compounds in classes 1, 2 and 3, respectively. Both RP and SVM models can be used for class 4 prediction. The inclusion of consensus analysis resulted in improved test set predictions for class 2 and 4 drugs.

CONCLUSIONS

The models can be used to predict BDDCS class for new compounds from molecular structure using readily available molecular descriptors and software, representing an area where in silico approaches could aid the pharmaceutical industry in speeding drugs to the patient and reducing costs. This could have significant applications in drug discovery to identify molecules that may have future developability issues.

Pharmacokinetics of the anti-human immunodeficiency virus agent 1-(beta-D-dioxolane)thymine in rhesus monkeys

Beta-D-dioxolane-thymine (D-DOT) has potent and selective in vitro activity against several clinically important resistant human immunodeficiency virus (HIV) mutants and is in advanced preclinical development. Therefore, the single-dose intravenous and oral pharmacokinetics of D-DOT were studied with three rhesus monkeys. The pharmacokinetic profiles of D-DOT in serum and urine were adequately described by a two-compartment open pharmacokinetic model. D-DOT was rapidly and almost completely absorbed (absorption rate constant = 2.7 h(-1); fraction of oral dose absorbed = 0.82 to 1.06). The average serum beta half-life was 2.16 h. The average central and steady-state volumes of distributions were 0.52 and 1.02 liter/kg of body weight, respectively, and the average systemic and renal clearance values were 0.36 liter/h/kg and 0.18 liter/h/kg. Four or eight percent of administered D-DOT was eliminated in the urine as glucuronide within 8 h after intravenous or oral administration, respectively. D-DOT reached levels in the cerebrospinal fluid in excess of 10 to 20 times the median effective concentration for wild-type HIV and resistant mutants. The potent antiretroviral activity of D-DOT against a lamivudine- and zidovudine-resistant HIV-1 mutant, together with an excellent pharmacokinetic profile for rhesus monkeys, suggest that further development is warranted.

Pharmacokinetic evaluation of emtricitabine in combination with other nucleoside antivirals in healthy volunteers

Emtricitabine is a potent nucleoside reverse transcriptase inhibitor approved as a once-daily drug in combination with other antiretroviral agents for the treatment of HIV infection. Several phase I studies were conducted in healthy volunteers over the course of clinical development to evaluate whether pharmacokinetic drug-drug interactions exist between emtricitabine and other nucleoside antivirals that are extensively eliminated by renal excretion. Potential interactions with stavudine and famciclovir were evaluated in single-dose studies, whereas interactions with zidovudine and its major metabolite, zidovudine glucuronide, were evaluated in a multiple-dose study. Plasma pharmacokinetic profiles and, in some studies, urinary excretion data were evaluated when each drug was administered alone and in combination with emtricitabine. Safety and plasma pharmacokinetic profiles of each drug administered alone or with emtricitabine were consistent with historical data. Statistical analyses indicated that there were no significant interactions between emtricitabine and these 3 nucleoside antivirals.

A method to distinguish between the de novo induction of thymidine kinase mutants and the selection of pre-existing thymidine kinase mutants in the mouse lymphoma assay

The mouse lymphoma assay (MLA) is the most widely used in vitro mammalian gene mutation assay. It detects various mutation events involving the thymidine kinase (Tk) gene in L5178Y/Tk+/- -3.7.2C mouse lymphoma cells. Mutants are detected using a thymidine analogue that arrests the growth of cells containing a functional Tk gene. However, there are a number of potential test chemicals that are thymidine analogues, and there is a problem when using the MLA to evaluate the mutagenicity of these chemicals. Thymidine analogues are activated by Tk before eliciting their toxicity. Therefore, any pre-existing Tk-/- mutants may avoid the toxicity of the test chemical and obtain a growth advantage over the Tk+/- cells, increasing the Tk mutant frequency (MF) in the culture via a selection mechanism. This potential mutant selection effect needs to be distinguished from de novo mutant induction in order to properly evaluate the mutagenicity of these chemicals. Here we describe a simple MLA study design that can differentiate between the selection of pre-existing mutants and de novo mutant induction. Trifluorothymidine (TFT), a thymidine analogue and the selection agent normally used in the MLA, and 4-nitroquinoline-1-oxide (4-NQO), a potent mutagen, were used to treat cells from two different Tk+/- mouse lymphoma cell cultures with different background MFs (approximately 112 and 305x10(-6)). Both agents significantly increased the Tk MFs in both the normal and high background cultures (p<0.01). In 4-NQO-treated cultures, the induced MFs (MF of treated culture-MF of control) for the cultures with different background MFs were about the same (p>0.1), while in TFT-treated cultures, they were significantly different (p<0.01). In TFT-treated cultures, the fold-increases of MF (MF of treated culture/MF of control) for the cultures with different background MFs were about the same (p>0.1), while in 4-NQO-treated cultures, they were significantly different (p<0.01). This study confirms that, when de novo mutations are induced, the induced MF is the same for cultures with normal and artificially high background MFs. In situations where the increase in MF is due solely to selection of pre-existing mutants, the "induced" MF will be a multiple of the background MF and the magnitude of the increase of the induced MF will depend upon the magnitude of the background MF. Our results demonstrate that it is possible, using this experimental design, to distinguish between chemicals acting primarily via the selection of pre-existing Tk mutants and those inducing de novo mutants in the MLA.

Antiretroviral therapy 2006: Pharmacology, applications, and special situations

As we approach the completion of the first 25 years of the human immunodeficiency virus (HIV) epidemic, there have been dramatic improvements in the care of patients with HIV infection. These have prolonged life and decreased morbidity. There are twenty currently available antiretrovirals approved in the United States for the treatment of this infection. The medications, including their pharmacokinetic properties, side effects, and dosing are reviewed. In addition, the current approach to the use of these medicines is discussed. We have included a section addressing common comorbid conditions including hepatitis B and C along with tuberculosis.

Pharmacokinetic properties of nucleoside/nucleotide reverse transcriptase inhibitors

Options for antiretroviral therapy in patients infected with HIV continue to expand as new drugs are integrated into treatment regimens. Nonetheless, nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs/NtRTIs) remain the backbone of highly active antiretroviral therapy (HAART). With the approval of emtricitabine in 2003, there are now 8 Food and Drug Administration (FDA)-approved NRTIs/NtRTIs. Several of these agents are effective as once-daily therapy, including didanosine, lamivudine, extended-release stavudine (FDA approved, but not currently available), tenofovir DF, and emtricitabine. Recent results from pharmacokinetic and clinical trials indicate that another NRTI, abacavir, may also be effective as a once-daily therapy, and FDA approval of once-daily dosing is anticipated. NRTIs are inactive as administered, requiring anabolic phosphorylation within target cells to achieve their antiretroviral effects. All NRTIs are converted to nucleoside triphosphates, which serve as the active metabolites (the NtRTI, tenofovir DF, only requires conversion to the diphosphate form). Frequency of drug administration is closely related to the pharmacokinetic properties of a drug. The key parameter is the half-life; however, the plasma elimination half-life of the NRTIs/NtRTIs as administered is of little use in developing a dosing schedule. Rather, the intracellular half-life of the nucleoside triphosphate is the relevant parameter. This article reviews the pharmacokinetic properties, particularly those of the various phosphorylation steps, of the NRTIs/NtRTIs.

Classification of orally administered drugs on the World Health Organization Model list of Essential Medicines according to the biopharmaceutics classification system

Since its inception in 1995, the biopharmaceutical classification system (BCS) has become an increasingly important tool for regulation of drug products world-wide. Until now, application of the BCS has been partially hindered by the lack of a freely available and accurate database summarising solubility and permeability characteristics of drug substances. In this report, orally administered drugs on the Model list of Essential Medicines of the World Health Organization (WHO) are assigned BCS classifications on the basis of data available in the public domain. Of the 130 orally administered drugs on the WHO list, 61 could be classified with certainty. Twenty-one (84%) of these belong to class I (highly soluble, highly permeable), 10 (17%) to class II (poorly soluble, highly permeable), 24 (39%) to class III (highly soluble, poorly permeable) and 6 (10%) to class IV (poorly soluble, poorly permeable). A further 28 drugs could be provisionally assigned, while for 41 drugs insufficient or conflicting data precluded assignment to a specific BCS class. A total of 32 class I drugs (either certain or provisional classification) were identified. These drugs can be further considered for biowaiver status (drug product approval based on dissolution tests rather than bioequivalence studies in humans).

Use of highly active antiretroviral therapy in patients with renal insufficiency

Antiretroviral agents, especially nucleoside reverse transcriptase inhibitors, require significant dosage adjustments in patients who have renal dysfunction and the human immunodeficiency virus (HIV). Some antiretroviral agents and fixed combination preparations are contraindicated in this population. In addition, many preferred antiretroviral regimens may be difficult to administer conveniently in patients with decreased creatinine clearance or in those receiving renal replacement therapies. Some highly active antiretroviral therapy regimens, however, can be used conveniently in patients with HIV and altered renal function.

Pharmacotherapy of perinatal HIV

Continued spread of HIV infection among women has led to the use of antiretrovirals in pregnant women and their newborns. Regional strategies to prevent mother-to-child transmission are evolving. Altered drug disposition during pregnancy may require altered dosing or 'boosted' therapies to avoid treatment failure. Maturing drug elimination pathways in newborns must also be considered for effective therapy. Potential teratogenic effects and increased sensitivity to antiretroviral toxicities might be encountered in this population. Use of highly active antiretroviral therapy (HAART) to suppress viral replication combined with formula feeding can reduce the rate of mother-to-child HIV transmission to less than 2%. In resource-limited settings, less intensive regimens including zidovudine, lamivudine and nevirapine still substantially reduce mother-to-child transmission. Although difficult to perform, clinical trials to determine the safety, pharmacokinetics and optimal dosing of antiretroviral in pregnant women and their newborns are urgently needed.

Stavudine Entry into Cerebrospinal Fluid After Single and Multiple Doses in Patients Infected with Human Immunodeficiency Virus

STUDY OBJECTIVE

To establish the pharmacokinetics of stavudine within the cerebrospinal fluid (CSF) of patients infected with human immunodeficiency virus (HIV).

DESIGN

Pharmacokinetic study.

SETTING

General clinical research center.

PATIENTS

Thirty-six patients infected with HIV; 21 were receiving long-term stavudine therapy, 15 were not (single-dose treatment group).

INTERVENTION

After an overnight fast, all patients received a single dose of stavudine 40 mg. Fifteen patients in the long-term treatment group and all 15 patients in the single-dose treatment group were randomized to undergo lumbar puncture 2, 4, or 6 hours after dosing (five patients for each time point from each group). The six other patients in the long-term treatment group underwent lumbar puncture 0 or 8 hours after dosing.

MEASUREMENTS AND MAIN RESULTS

Serum stavudine concentrations were obtained just before dosing, 1 hour after dosing (approximate peak), and at the time of lumbar puncture. The CSF was also analyzed for cell counts, protein, and glucose levels. The mean peak serum stavudine concentration in the long-term treatment group was estimated to be 580.7 ng/ml (2.59 micromol/L), occurring approximately 1.3 hours after dosing. The CSF concentrations over 0-8 hours were 0.0-109.9 ng/ml (0.00-0.49 micromol/L) with an overall mean of 51.6 ng/ml (0.23 micromol/L). Mean peak CSF concentration was estimated to be 62.8 ng/ml (0.28 micromol/L), occurring 4.7 hours after dosing. For the 15 patients not taking stavudine, both the serum and the CSF estimated peaks were significantly lower than those of the long-term group: 475.3 ng/ml (2.12 micromol/L) and 40.4 ng/ml (0.18 micromol/L), respectively. However, time to peak was similar at 1.2 hours and 5.0 hours, respectively. In both groups, no correlation was found between CSF and baseline or peak serum stavudine concentrations, CSF white blood cell count, baseline CD4 + lymphocyte count, or plasma viral load.

CONCLUSION

Mean CSF stavudine concentrations equaled or exceeded the mean concentration producing 50% of the maximal effect in vivo (EC 50 ) for HIV. The CSF concentrations were higher in the stavudine-experienced patients, indicating that concentrations rise with progressive doses until steady state is reached.

Antiretroviral drug pharmacokinetics in hepatitis with hepatic dysfunction

Chronic viral hepatitis is common among persons with HIV-1 infection, because of shared modes of transmission, and coinfection results in accelerated liver damage, compared with persons with chronic viral hepatitis alone. The use of highly active antiretroviral therapy (HAART) has led to a significant decrease in the morbidity and mortality associated with HIV-1 infection. A number of the medications that are commonly used in HAART regimens are metabolized by the hepatic CYP enzymes, which raises the possibility of significant interactions between antiretroviral medications and hepatic impairment induced by chronic viral hepatitis. Although the data are still very scant, the pharmacokinetics of several antiretroviral medications have been shown to be significantly altered in the presence of liver disease. In the present report, we review the available data and consider potential options, such as dose adjustment and therapeutic drug monitoring, for the administration of antiretroviral therapy to patients with significant hepatic impairment.

Ganciclovir nucleotides accumulate in mitochondria of rat liver cells expressing the herpes simplex virus thymidine kinase gene

BACKGROUND

Ganciclovir exhibits broad-spectrum activity against DNA viruses such as cytomegaloviruses, herpes simplex viruses, varicella-zoster virus, Epstein-Barr virus and human herpes virus-6. Ganciclovir is widely applied for anti-herpetic treatment, cytomegalovirus prophylaxis after organ transplantation, and, more recently, in experimental gene therapy to eradicate cycling cells that express the herpes simplex virus thymidine kinase gene. Although ganciclovir supposedly acts as a chain terminator, there is compelling evidence demonstrating the presence of ganciclovir, but not of acyclovir, incorporated internally into DNA, leaving the precise mechanism by which ganciclovir inhibits DNA synthesis enigmatic.

METHODS

To study the potential involvement of mitochondria in the ganciclovir nucleotide cytotoxicity, we used adenovirus-mediated gene transfer to express herpes simplex virus thymidine kinase in rat liver and administered ganciclovir 2 days post-infection. The integrity and function of mitochondria in the rat liver cells were evaluated by several techniques. In addition, we analyzed the nucleotide pools in cellular extracts and in isolated mitochondria.

RESULTS

We show that ganciclovir nucleotides are abundantly present in the mitochondria of rat livers that express the HSVtk gene. Already 48 h after administration, 10-30% of the total mitochondrial nucleotide pool consists of ganciclovir nucleotides. Their presence is correlated with a lower amount of mitochondrial DNA, a reduced mitochondrial-membrane potential, morphological abnormalities, and liver dysfunction.

CONCLUSIONS

These data provide evidence for the involvement of mitochondria in the hepatotoxicity of the HStk/ganciclovir combination. This may explain the toxicity of the HSVtk/gancilovir combination in some metabolically active but non-proliferating cells, such as liver cells. This toxicity limits the applicability of this enzyme/prodrug combination.

Pharmacokinetic Evaluation and Short-Term Activity of Stavudine, Nevirapine, and Nelfinavir Therapy in HIV-1???Infected Adults

OBJECTIVE

Evaluate pharmacokinetic interaction, short-term safety, and antiretroviral activity of stavudine (d4T), nevirapine (NVP), and nelfinavir (NFV) as combination HIV-1 therapy.

DESIGN

Prospective, open-label study investigating the pharmacokinetic interactions between d4T, NVP, and NFV and documenting short-term tolerability and virologic and immunologic activity.

METHODS

Twenty-five HIV-1-infected adults, naive to nonnucleoside reverse transcriptase inhibitors (NNRTIs) and protease inhibitors (PIs), < or = 6 months of d4T treatment, CD4 > or = 100 cells/mm, and viral load > = 5,000 copies/mL enrolled. All received NFV 750 mg 3 times daily and d4T 30-40 mg twice daily for 1 week, then added NVP at 200 mg once daily for 2 weeks and 200 mg twice daily thereafter. Steady-state pharmacokinetic parameters of NFV, AG1402 (metabolite of NFV), and d4T were compared before and after the addition of NVP.

RESULTS

No statistically significant changes in NFV or d4T pharmacokinetics were observed following the addition of NVP. Levels of AG1402 were suppressed 60-70%. Drug-related adverse events were seen at expected rates. At day 36, median viral load suppression was 2.0 log10 and absolute CD4 count increased by 111 cells/mm.

CONCLUSIONS

NVP administration did not significantly affect the steady-state pharmacokinetic parameters of NFV or d4T. The combination of d4T, NVP, and NFV induced rapid suppression of HIV-1 viral load and rises in CD4 cell count.

Intermittent administration of high-dose stavudine to nucleoside-experienced individuals infected with HIV-1

To explore a novel method to use nucleosides in heavily treated patients, 4-week cycles of high-dosage (280 mg/d) stavudine were given to 11 asymptomatic patients who had previously received an average of 6 years of nucleoside reverse transcriptase inhibitors (NRTIs). The stavudine dose was targeted to produce a calculated steady-state concentration of 1.5 microM (336 ng/mL). Participants received stavudine for the first 4 weeks, after which it was discontinued for 4 weeks. Additional 4-week drug cycles were given if plasma HIV-1 RNA levels increased to at least 75% of baseline values. The 11 subjects received 38 cycles (average, 3.5 cycles per subject) during the 44-week study. Stavudine was well tolerated. Considering all data, there was a median 0.65 log(10) reduction in viral load as well as a median increase in CD4 cell count of 110/mm3 at the end of the cycles. However, plasma viremia increased and CD4 cell counts decreased between cycles. Viral load and CD4 cell responses were similar for up to 4 successive cycles. This suggested that increasing viral resistance was not a problem, as reflected by the acquisition of only one new nucleoside reverse transcriptase mutation among the participants. Significant relationships between stavudine exposure and changes in plasma HIV RNA levels were observed. A similar approach might be considered using a more potent regimen for patients in whom resistance to nucleosides is a major reason for therapeutic failure.

A new sensitive cartridge-RIA method for determination of stavudine (D4T) triphosphate in human cells in vivo

We describe a simple and sensitive method to determine stavudine triphosphate, the active intracellular anabolite of stavudine (D4T). Quantification of D4T triphosphate was performed with a combined cartridge-radioimmunoassay (cartridge-RIA) which enabled us to measure concentrations of D4T triphosphate as low as 0.5 ng/ml, or an intracellular concentration which corresponds to 20 fmol/10(6) cells if diluted like our previously published zidovudine (ZDV) assay. The only alternate methodology at present employs liquid chromatography mass spectroscopy (LC-MS/MS). The use of the cartridge-RIA methodology provides a cost-effective alternative for the determination of in vivo cellular pharmacokinetics studies of D4T in human immunodeficiency virus (HIV)-infected persons.

Determination of dosing guidelines for stavudine (2',3'-didehydro-3'-deoxythymidine) in children with human immunodeficiency virus infection

The results of the development of dosing guidelines for stavudine in human immunodeficiency virus (HIV)-infected children are summarized. Included in the integrated analyses were 21 and 33 HIV-infected pediatric and adult patients, respectively, from three phase I-II studies. Data for 21 children and 18 adults who received intravenous doses of 0.125 to 2 and 0.5 to 1 mg/kg of body weight, respectively, were used for the determination of dosing guidelines; exposure data for 16 children and 15 adults who received oral doses of 1 to 2 and 0.5 to 1 mg/kg/day, respectively, were used to validate the dosing recommendations for children. Significant relationships were observed between total body clearance (in milliliters per minute) in children and adults combined and demographic parameters of age, body weight, and body surface area (R(2) = 0.77 to 0.80; P = 0.0001). Models of approximated pediatric dose based on clearance values and direct adult exposure yielded a stavudine dosage of 2 mg/kg/day for children of < or =30 kg of body weight and 1 mg/kg/day (adult dose) for children of >30 kg of body weight.

Determination of pharmacokinetic parameters of stavudine in Japanese patients infected with HIV-1, using a Gaussian-like input rate function

We sought to examine variation in pharmacokinetic parameters of stavudine in Japanese patients infected with HIV-1. Stavudine concentrations were measured in two hemophiliacs (HIV-1 asymptomatic carriers (ACs)) and two non-hemophiliacs (both ACs). To simulate the plasma stavudine concentrations following a single oral dose, we used a Gaussian-like input rate function in a single compartment model. The theoretical equation successfully simulated changes in the time course of plasma stavudine concentrations in all four patients. The mean+/-SD of Tmax, Cmax, AUC0-infinity, and t 1/2 were 0.96+/-0.26 hr, 478+/-90 ng/mL, 1112+/-136 ng x hr/mL, and 1.26+/-0.23 hr, respectively. Pharmacokinetic characteristics were comparable to those reported in the literature, although the mean of Cmax was slightly lower and means of Tmax and t 1/2 slightly longer than those previously reported. Interpatient variability in plasma stavudine concentrations was not as dramatic as that seen with lamivudine. Thus, we confirmed the validity of the uniform therapeutic regimen of stavudine in Japanese people with HIV-1 infection.

Antiretroviral therapy 2000

As we enter the new millennium, there have been dramatic improvements in the care of patients with HIV infection. These have prolonged life and decreased morbidity and mortality. There are fourteen currently available antiretrovirals approved in the United States for the treatment of this infection. The medications, including their pharmacokinetic properties, side effects, and dosing are reviewed. In addition, the current approach to the use of these medicines is discussed.

Determination of stavudine in human plasma and urine by high-performance liquid chromatography using a reduced sample volume

Sensitive high-performance liquid chromatographic assays have been developed for the quantification of stavudine (2',3'-didehydro-3'-deoxythymidine, d4T) in human plasma and urine. The methods are linear over the concentration ranges 0.025-25 and 2-150 microg/ml in plasma and urine, respectively. An aliquot of 200 microl of plasma was extracted with solid-phase extraction using Oasis cartridges, while urine samples were simply diluted 1/100 with HPLC water. The analytical column, mobile phase, instrumentation and chromatographic conditions are the same for both methods. The methods have been validated separately, and stability tests under various conditions have been performed. The detection limit is 12 ng/ml in plasma for a sample size of 200 microl. The bioanalytical assay has been used in a pharmacokinetic study of pregnant women and their newborns.

Mechanism of inhibition of the human immunodeficiency virus type 1 reverse transcriptase by d4TTP: an equivalent incorporation efficiency relative to the natural substrate dTTP

Among the clinically used nucleoside analogue inhibitors that target human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT), there is little detailed mechanistic information on the interactions of 2',3'-didehydro-2', 3'-dideoxythymidine-5'-triphosphate (d4TTP) with the enzyme. primer-template complex and how these interactions compare with those of the natural substrate, dTTP. Using a pre-steady-state kinetic analysis, we found that d4TTP was incorporated by HIV-1 RT just as efficiently as dTTP during both DNA- and RNA-dependent DNA synthesis. To our knowledge, these results represent the first observation of a 3'-modified nucleoside triphosphate analogue that has an incorporation efficiency comparable to that observed for the natural substrate during DNA synthesis by HIV-1 RT. This information provides a mechanistic basis for understanding the inhibition of HIV-1 RT by d4TTP as well as insight into the clinically observed lack of d4T resistance mutations in HIV-1 RT isolated from AIDS patients.

Pharmacokinetics of [(14)C]abacavir, a human immunodeficiency virus type 1 (HIV-1) reverse transcriptase inhibitor, administered in a single oral dose to HIV-1-infected adults: a mass balance study

Abacavir (1592U89) ((-)-(1S, 4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-2-cyclopentene- 1-m ethanol) is a 2'-deoxyguanosine analogue with potent activity against human immunodeficiency virus (HIV) type 1. To determine the metabolic profile, routes of elimination, and total recovery of abacavir and metabolites in humans, we undertook a phase I mass balance study in which six HIV-infected male volunteers ingested a single 600-mg oral dose of abacavir including 100 microCi of [(14)C]abacavir. The metabolic disposition of the drug was determined through analyses of whole-blood, plasma, urine, and stool samples, collected for a period of up to 10 days postdosing, and of cerebrospinal fluid (CSF), collected up to 6 h postdosing. The radioactivity from abacavir and its two major metabolites, a 5'-carboxylate (2269W93) and a 5'-glucuronide (361W94), accounted for the majority (92%) of radioactivity detected in plasma. Virtually all of the administered dose of radioactivity (99%) was recovered, with 83% eliminated in urine and 16% eliminated in feces. Of the 83% radioactivity dose eliminated in the urine, 36% was identified as 361W94, 30% was identified as 2269W93, and 1.2% was identified as abacavir; the remaining 15.8% was attributed to numerous trace metabolites, of which <1% of the administered radioactivity was 1144U88, a minor metabolite. The peak concentration of abacavir in CSF ranged from 0.6 to 1.4 microg/ml, which is 8 to 20 times the mean 50% inhibitory concentration for HIV clinical isolates in vitro (0.07 microg/ml). In conclusion, the main route of elimination for oral abacavir in humans is metabolism, with <2% of a dose recovered in urine as unchanged drug. The main route of metabolite excretion is renal, with 83% of a dose recovered in urine. Two major metabolites, the 5'-carboxylate and the 5'-glucuronide, were identified in urine and, combined, accounted for 66% of the dose. Abacavir showed significant penetration into CSF.

Effects of liver disease on pharmacokinetics. An update

Liver disease can modify the kinetics of drugs biotransformed by the liver. This review updates recent developments in this field, with particular emphasis on cytochrome P450 (CYP). CYP is a rapidly expanding area in clinical pharmacology. The information currently available on specific isoforms involved in drug metabolism has increased tremendously over the latest years, but knowledge remains incomplete. Studies on the effects of liver disease on specific isoenzymes of CYP have shown that some isoforms are more susceptible than others to liver disease. A detailed knowledge of the particular isoenzyme involved in the metabolism of a drug and the impact of liver disease on that enzyme can provide a rational basis for dosage adjustment in patients with hepatic impairment. The capacity of the liver to metabolise drugs depends on hepatic blood flow and liver enzyme activity, both of which can be affected by liver disease. In addition, liver failure can influence the binding of a drug to plasma proteins. These changes can occur alone or in combination; when they coexist their effect on drug kinetics is synergistic, not simply additive. The kinetics of drugs with a low hepatic extraction are sensitive to hepatic failure rather than to liver blood flow changes, but drugs having a significant first-pass effect are sensitive to alterations in hepatic blood flow. The drugs examined in this review are: cardiovascular agents (angiotensin converting enzyme inhibitors, angiotensin II receptor antagonists, calcium antagonists, ketanserin, antiarrhythmics and hypolipidaemics), diuretics (torasemide), psychoactive and anticonvulsant agents (benzodiazepines, flumazenil, antidepressants and tiagabine), antiemetics (metoclopramide and serotonin antagonists), antiulcers (acid pump inhibitors), anti-infectives and antiretroviral agents (grepafloxacin, ornidazole, pefloxacin, stavudine and zidovudine), immunosuppressants (cyclosporin and tacrolimus), naltrexone, tolcapone and toremifene. According to the available data, the kinetics of many drugs are altered by liver disease to an extent that requires dosage adjustment; the problem is to quantify the required changes. Obviously, this requires the evaluation of the degree of hepatic impairment. At present there is no satisfactory test that gives a quantitative measure of liver function and its impairment. A critical evaluation of these methods is provided. Guidelines providing a rational basis for dosage adjustment are illustrated. Finally, it is important to consider that liver disease not only affects pharmacokinetics but also pharmacodynamics. This review also examines drugs with altered pharmacodynamics.

P-glycoprotein, secretory transport, and other barriers to the oral delivery of anti-HIV drugs

Orally administered anti-HIV drugs must be adequately and consistently absorbed for therapy to be successful. This review discusses the barriers to achieving oral bioavailability for the currently available anti-HIV drugs. Most reverse transcriptase inhibitors have good oral bioavailabilities. Didanosine bioavailability could be reduced by acid instability, first-pass hepatic metabolism, and possibly poor intestinal permeation. Bioavailability of zidovudine is also reduced by first-pass metabolism. The non-nucleoside reverse transcriptase inhibitors have oral bioavailabilities most probably limited by poor aqueous solubility. For each of the currently marketed HIV protease inhibitors, solubility, intestinal permeability, and first-pass metabolism could contribute to reducing oral bioavailability. The intestinal permeabilities of these agents is influenced by secretory transport. In vitro, secretory transport, which appears to be P-glycoprotein-mediated, is much greater than permeation in the absorptive direction for indinavir, nelfinavir, ritonavir, and saquinavir. The mechanisms of secretory intestinal transport are reviewed, and the factors that may influence the impact of secretory transport in vivo are considered.

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.

New developments in the treatment of HIV disease: An overview

The management of HIV disease has evolved into a complicated and sophisticated subspecialty in recent years. Fourteen drugs, in various combinations, are being used in increasingly complex treatment regimens. The side effects of some of these drugs, as well as certain drug-drug interactions may mimic signs and symptoms of HIV disease itself. Therefore it is imperative for the emergency physician to be knowledgeable about the new medications as well as about selected adverse effect and drug interaction profiles in order to be able to take care of the increasing numbers of HIV-positive patients presenting to emergency departments. This article aims to provide a focused review of these topics. In addition, health care workers with significant exposures to HIV-infected body fluids may present to the emergency department for initial evaluation. This presents a situation whereby emergency physicians may have to prescribe appropriate combinations of antiretroviral agents themselves. Thus familiarity with the basic principles of post-exposure prophylaxis is desirable and current Centers for Disease Control and Prevention guidelines are briefly reviewed.

A multiple drug interaction study of stavudine with agents for opportunistic infections in human immunodeficiency virus-infected patients

The effects of multiple opportunistic infection medications on stavudine pharmacokinetics were evaluated. Ten patients with CD4 counts of less than 200 cells/mm3 received stavudine (40 mg twice daily) in combination with one to three other drugs used to treat opportunistic infections. Serial blood samples for stavudine concentrations were collected after 1 week of therapy on each regimen and assayed for stavudine by using a validated high-pressure liquid chromatography method. Although the maximum concentration of drug in serum was significantly decreased when the drug was given in combination with three opportunistic infection medications, the area under the concentration-time curve did not significantly differ across various treatment regimens. Stavudine exposure was not significantly altered by multiple concomitant medications. Side effects were minor throughout the 3-month study period. The tolerability of stavudine, combined with its lack of drug interactions, makes it an attractive agent for use as part of a combination regimen.