Pharmacokinetic optimisation of antiretroviral therapy in patients with HIV infection.

More than 7 years after the introduction of zidovudine for treatment of HIV infection, little use has been made of the pharmacokinetic properties of this or any of the subsequently approved antiretroviral agents to optimise therapy. This is partly because of the limits of technologies developed to measure clinically relevant forms and concentrations of these drugs, and partly because the clinical community has been slow to recognise the potential benefits of pharmacokinetic optimisation of nucleoside analogue therapy in any disease. Nonetheless, for some of these agents, progress in understanding the relationship between pharmacokinetics and pharmacodynamics has been made. With zidovudine, for example, even though plasma concentrations have little clinical utility, evidence suggests that concentrations of active phosphorylated forms of zidovudine inside target cells are related to disease progression and toxicity. Furthermore, a decreased ability to phosphorylate zidovudine might be a prerequisite for the emergence of zidovudine-resistant HIV strains. Measurements of phosphorylated zidovudine inside cells similarly suggest that 100 mg of oral zidovudine every 8 hours approximates the optimal initial dosage regimen in asymptomatic patients. Increased plasma didanosine concentrations have been associated with several measures of clinical improvement in patients, and may be associated with an increased risk of toxicity as well. For zalcitabine and stavudine, however, the picture is much less clear. Their pharmacokinetic and pharmacodynamic relationships have not been studied in patients. Furthermore, there is insufficient data on the effects of age, gender, race and concurrent underlying conditions on the pharmacokinetics of all of these agents. Mounting evidence suggests that monitoring of these compounds could lead to individually optimised intervention strategies. Given the marginal benefits of therapy with these agents, their proven toxic effects and the lack of proven alternatives, it is critical that the clinical community strive to make the most effective use of these agents in the treatment of their patients.

Pharmacokinetics of zidovudine phosphorylation in peripheral blood mononuclear cells from patients infected with human immunodeficiency virus.

As part of an effort towards optimization of dosing of zidovudine (ZDV), formation and elimination of total phosphorylated ZDV (ZDVPt) in peripheral blood mononuclear cells were examined in 21 asymptomatic human immunodeficiency virus-infected patients during their first 24 weeks of therapy (AIDS Clinical Trials Group Protocol 161). Intracellular concentrations of ZDVPt were measured with a previously described and validated radioimmunoassay technique. Although ZDV phosphorylation occurred readily upon initiation of therapy, it declined with time; the area under the concentration-time curve (AUC) at week 4 (mean +/- standard deviation, 3.41 +/- 0.93 pmol.h/10(6) cells) was significantly greater than that at week 24 (2.19 +/- 1.10 pmol.h/10(6) cells). Plasma ZDV AUC did not change with time and did not correlate with ZDVPt AUC. In dose-response experiments (20 to 100 mg orally), phosphorylation did not proportionally increase with increasing plasma ZDV concentrations. Similarly, compared with a single dose, two doses of ZDV over an 8-h period resulted in little ZDVPt increase in cells relative to increase in plasma ZDV concentrations. The half-life of intracellular ZDVPt was twice that of plasma ZDV (4 versus 2 h), suggesting that an every-8-h dosing regimen is justifiable. These findings suggest that metabolism of ZDV to its active intracellular forms may be saturable in some patients, is poorly correlated with plasma concentrations, and diminishes over time. These findings have implications for future development and management of anti-human immunodeficiency virus nucleoside therapy.

Correlates of zidovudine phosphorylation with markers of HIV disease progression and drug toxicity.

OBJECTIVE: To determine the relationships between in vivo zidovudine (ZDV) phosphorylation in cells from HIV-infected patients and markers associated with disease progression and drug toxicity. DESIGN: A pharmacokinetic study of ZDV metabolism sponsored by the AIDS Clinical Trials Group (protocol 161). Plasma and intracellular pharmacokinetics following a 100 mg oral dose of ZDV were determined at weeks 4 and 24 of initial therapy in adult patients. Plasma concentrations and phosphorylated ZDV were determined by radioimmunoassay, and area under the concentration-time curves (AUC) were compared with clinical data collected during the pharmacokinetic study. SETTING: An outpatient setting at the University of Cincinnati AIDS Treatment Center, Cincinnati, Ohio, USA. PATIENTS: HIV-infected adults with CD4+ lymphocyte counts 200-500 x 10(6) cells/l with no prior history of anti-HIV therapy and no active infections requiring systemic therapy. Of 30 patients enrolled, 21 were evaluable. INTERVENTIONS: None. MAIN OUTCOME MEASURES: AUC of plasma ZDV and intracellular total phosphorylated ZDV were compared with change from baseline of the following surrogate markers: CD4+ lymphocyte count, %CD4+ lymphocytes, CD4+/CD8+ cell ratio, serum beta 2-microglobulin, serum neopterin, neutrophils, red cell count, and hemoglobin. RESULTS: No correlations between plasma AUC and markers of therapeutic response were observed. However, significant positive correlations were observed between the AUC of total phosphorylated ZDV and changes in the %CD4+ lymphocytes and CD4+/CD8+ lymphocyte ratio; a negative correlation was observed with change in hemoglobin. Patients who responded to ZDV therapy, as measured by these variables, demonstrated significantly higher intracellular AUC (> 3 pmol x h/10(6) cells) than those who did not (approximately 2 pmol x h/10(6) cells). CONCLUSIONS: The ability of HIV-infected patients to phosphorylate ZDV correlates with changes in markers associated with drug effect and toxicity. Potential individualization of therapy through monitoring of total phosphorylated ZDV in patients therefore warrants further exploration.

Pharmacokinetics of zidovudine phosphorylation in patients infected with the human immunodeficiency virus.

The relationship between zidovudine phosphorylation inside mononuclear cells and plasma zidovudine pharmacokinetic was assessed in six subjects. Plasma and intracellular concentrations were measured by radioimmunoassay over an 8-h period after administration of 100 or 200 mg of zidovudine. Plasma pharmacokinetics followed expected patterns, with considerable interpatient variability in area under the concentration-versus-time curve (AUC), and a terminal half-life of 1.5 h. Intracellular AUC was even more variable than plasma AUC, but the data suggested a crude linear relationship between these parameters. The intracellular half-life of 3.5 h was consistently longer than the plasma half-life, and varied little between patients. The prolonged intracellular half-life suggested that total phosphorylated zidovudine, as measured by the method described, is not greatly dominated by the 5'-monophosphate as predicted from the in vitro studies reported in the literature. Plasma concentrations of zidovudine have shown little correlation with clinical effect. Study of the relationship between phosphorylated zidovudine and clinical outcome could lead to a more effective management of therapy.

Concentrations of phosphorylated zidovudine (ZDV) in patient leukocytes do not correlate with ZDV dose or plasma concentrations.

Zidovudine (ZDV) elicits its antiviral effect through intracellular metabolism to the 5'-triphosphate, which interferes with viral replication. Monitoring of the active metabolites of ZDV in cells could lead to an intracellular therapeutic range. This study was performed to determine whether a radioimmunoassay, previously used for in vitro quantitation of total phosphorylated ZDV inside peripheral blood leukocytes, could be used for similar determinations in patient samples. The relationship between ZDV dose, plasma concentrations, and intracellular metabolite concentrations was also examined. Ten-milliliter blood samples were drawn from each of 13 human immunodeficiency virus-infected patients and were assayed. Intracellular concentrations of phosphorylated ZDV ranged from 0.33 to 3.54 pmol/10(6) cells, similar to those observed in vitro. Phosphorylated ZDV was independent of dose, and did not correlate with plasma concentrations. Intracellular concentration in the patient population as a whole did not change during the 4-h dosing interval, while plasma concentration decayed normally. Later determinations in the same patients gave intracellular values within 31% of earlier values. Intraassay variability was less than 10%. Thus, the method is valid for measurement of phosphorylated ZDV in patient cells. Although individual concentrations showed no clear change during the 3-month study period, intracellular concentrations decreased with increasing length of therapy (up to 3 years) in the population as a whole. This suggests a decreased cellular ability to phosphorylate ZDV after prolonged exposure to drug. The lack of intracellular decay implies a half-life longer than the 1-h half-life of plasma ZDV. These data suggest that smaller doses or longer dosing intervals might maintain intracellular concentrations once steady state is achieved.(ABSTRACT TRUNCATED AT 250 WORDS)

State-of-the-art of zidovudine monitoring.

Zidovudine, the only drug approved by the FDA for the treatment of AIDS, has well-documented efficacy, but numerous toxic side effects. Despite considerable knowledge of the pharmacology of the drug, however, a therapeutic concentration range has not been established. This article reviews methodologies used for measurement of zidovudine in serum. Procedures for several methods are described and comparisons offered. In addition to methodologies, other issues of concern in establishing or maintaining a zidovudine monitoring service are also discussed, including the clinical relevance of routine zidovudine monitoring, method-related quality control, and laboratory safety. Methodologies currently being developed to measure the active intracellular metabolites of zidovudine are also described. These methods are believed to be the key to fully understanding the relationships between zidovudine pharmacology, toxicity, and efficacy.

In vitro measurement of phosphorylated zidovudine in peripheral blood leucocytes.

A method has been developed to measure the concentration of total phosphorylated zidovudine (ZDV) inside peripheral blood leucocytes (PBLs) using a modified commercial radioimmunoassay (RIA) specific for ZDV. ZDV 5'-monophosphate was readily synthesized and used as a procedural control for RIA modification. PBLs were isolated from healthy volunteers and incubated with ZDV for 24 h to allow metabolic phosphorylation. Viable cells were counted, washed, and extracted overnight with 60% methanol. After evaporation, the extract was reconstituted in Tris buffer, pH 9.5. Because of minimal RIA antibody cross reactivity with phosphorylated ZDV, samples were split into two fractions, one of which was treated with alkaline phosphatase (AP) to liberate phosphate groups. Each fraction was then assayed for ZDV. Concentrations of phosphorylated ZDV were determined by subtracting the concentration of the non-AP-treated fraction from that of the treated fraction. Recovery of phosphorylated ZDV from cell extracts was approximately 90%, and reproducibility was acceptable (coefficients of variation less than 15% for concentrations greater than or equal to 0.25 ng/ml). Intracellular concentrations (0.1-1.4 pmoles/10(6) cells) followed a nonlinear dose-response relationship over the range 0-50 microM extracellular ZDV, with concentration-dependent saturation. These results demonstrate the feasibility of determining concentrations of phosphorylated ZDV in HIV-infected patients, a potentially key step in establishing a therapeutic range and optimal dosing regimen for these patients.