Abstract
Racemic dOTC (BCH-10652) is a novel nucleoside reverse transcriptase inhibitor consisting of two enantiomers of 2′-deoxy-3′-oxa-4′-thiocytidine, (−)dOTC and (+)dOTC, that have both shown activity against human immunodeficiency virus type 1. The objectives of this study were to characterize the safety, tolerability, and stereospecific pharmacokinetics of single oral doses of racemic dOTC in healthy, nonsmoking adult male volunteers. Subjects received single oral doses of 100, 200, 400, 800, and 1,600 mg of racemic dOTC in a placebo-controlled, dose-rising, incomplete crossover study design, and the pharmacokinetics of both (+)dOTC and (−)dOTC were determined. At least six subjects were studied at each dose level, with each subject studied in three of five periods, receiving two different doses of racemic dOTC and one placebo dose. Plasma and urine drug concentrations were measured for 24 to 48 h after each dose. Pharmacokinetic models were fitted to the plasma concentrations of (+)dOTC and (−)dOTC using maximum likelihood and maximum a posteriori Bayesian procedures. Statistical hypothesis testing was by nonparametric analysis of variance (where possible) and, when tests with dose as a covariate were performed, by linear mixed-effects modeling. The mean terminal elimination half-lives for (+)dOTC and (−)dOTC were 15.3 h (coefficient of variation [CV], 28%) and 11.3 h (CV, 43%), respectively (P < 0.05). The mean CV for total oral clearance (liter/h/65 kg) was 17.5 (25%) for (+)dOTC and 21.5 (24%) for (−)dOTC; for oral steady-state volume of distribution (liter/65 kg), values were 61.8 (24%) for (+)dOTC and 34.1 (33%) for (−)dOTC (P < 0.05). The mean CV for renal clearance (liter/h/65 kg) of (+)dOTC was 10.4 (19%) and for (−)dOTC was 13.6 (20%) (P < 0.05). There was no significant effect of dose size on the pharmacokinetics of racemic dOTC. All doses were well tolerated, and no serious adverse events or laboratory abnormalities were observed.
Progress in the fight against human immunodeficiency virus (HIV) infection has proceeded at a rapid pace in recent years, with the number of AIDS-related deaths between 1994 and 1998 decreasing from 33 to 5 per 100 person years (2). A better understanding of the epidemiology, pathogenesis, and virology of HIV and AIDS has led to the introduction of treatment and prevention strategies that have effectively altered the natural course of this disease. It is now understood that an effective treatment regimen requires multiple drugs in combination, which often leads to complex, inconvenient, and sometimes intolerable or serious medication-related side effects. Clearly, new agents that are more effective, less toxic, and more convenient are necessary to further improve the medical care of HIV-infected patients.
Racemic dOTC (BCH-10652) is a novel reverse transcriptase inhibitor of the 4′-thio heterosubstituted class of nucleoside analogues and is a racemic mixture of the enantiomers of 2′-deoxy-3′-oxa-4′-thiocytidine. Both enantiomers, (−)dOTC and (+)dOTC, exhibit equipotent activities against HIV type 1 (HIV-1), with mean 50% inhibitory concentrations against wild-type clinical isolates reported as 1.76 μM and against clinical isolates resistant to 3TC and AZT as approximately 2.5 μM (6). The purpose of this first-time study with humans was to investigate the safety, tolerability, and pharmacokinetics of single oral doses of racemic dOTC in healthy adult male volunteers.
MATERIALS AND METHODS
The study protocol was approved by the Millard Fillmore Health Systems Institutional Review Board (Buffalo, N.Y.), and written informed consent was obtained for each subject prior to study participation. Racemic dOTC and identical-appearing placebo was supplied by BioChem Pharma Inc. (Laval, Quebec, Canada).
Study population.
The study participants were healthy, non-HIV-infected adult male volunteers. Subjects were nonsmokers between 18 and 50 years of age who had a total body weight of ≥50 kg and were within 10% of ideal body weight. Exclusion criteria included any clinically relevant abnormality identified during the screening physical or laboratory examination; a history of significant cardiac, renal, hepatic, neurologic, or hematologic abnormality; a history of alcohol or drug abuse within 6 months of the study; treatment with an investigational drug within 30 days prior to the first study session; use of prescription or nonprescription drugs within 1 week prior to or during the study; or donation of 1 U of blood within 60 days prior to the first dose of study medication.
Study design.
This was a randomized, single-dose, single-blind, placebo-controlled, dose-ranging, three-period incomplete crossover study. In five study periods, racemic dOTC was administered orally in capsules at doses of 100, 200, 400, 800, and 1,600 mg, in order from lowest to highest dose. At each dose level, six subjects received active drug, and three subjects received placebo. Each individual subject received two of the five doses of racemic dOTC and one dose of placebo throughout the entire study. Escalation to the next dose in the sequence was allowed only after safety and tolerability of the current dose level was demonstrated in at least three subjects.
Prior to each study period, subjects were admitted to the Clinical Research Center and were maintained in the fasting state, except for water, for at least 10 h before dosing. Subjects remained in the fasting state for at least 5 h after dosing. All doses were administered with 240 ml of tepid water. Blood samples for (−)dOTC and (+)dOTC serum concentration measurements were drawn prior to dosing (time zero) and at 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 4, 6, 8, 10, 12, 16, 20, and 24 h after dosing. Midway through the study, the plasma sampling strategy was extended to include samples at 36 and 48 h after dosing, and the 6- and 10-h samples were removed. All blood samples were immediately separated at 4°C by centrifugation and stored at approximately −20°C until analyzed. Urine samples were collected during the intervals of 0 to 4, 4 to 8, 8 to 12, 12 to 16, 16 to 20, and 20 to 24 h after dosing. The exact start and stop times of each urine collection period and the total volume were recorded; a 50-ml aliquot of urine from each interval was processed and stored for assay.
For drug assay, (−)dOTC and (+)dOTC were extracted from human plasma using a solid-phase extraction cartridge. Plasma drug concentrations of each enantiomer were measured by reverse-phase high-pressure liquid chromatography with UV detection, using 2′,3′-dideoxycytidine as an internal standard. The internal standard, (−)dOTC, and (+)dOTC had column retention times of approximately 15, 20, and 21.5 min, respectively. For assay accuracy, (−)dOTC had a coefficient of variation (CV) range of 2.4 to 4.5% for interassay and 1.0 to 4.2% for intraassay variability. For (+)dOTC, the CV range was between 2.4 and 3.9% for interassay and between 1.3 and 2.7% for intraassay variability. The lower limit of quantitation for both enantiomers was 3.0 ng/ml, and no interference from endogenous human plasma components was identified. The assay was linear over the range of 3.0 to 1,000 ng/ml for both enantiomers. Concentrations above 1,000 ng/ml were diluted to obtain a concentration within the linear portion of the calibration curve and were reanalyzed. Quantitation was performed using the peak height ratio method, and samples were assayed in random order.
Safety and tolerability.
All subjects underwent an initial-screening physical examination within 30 days prior to receiving the first dose of study medication. This examination included a complete medical history, physical examination, and 12-lead electrocardiogram (ECG). Continuous dual-lead ECG monitoring was performed for 1 h. Blood and urine specimens were obtained for standard clinical laboratory tests.
Prior to dosing, another physical examination was performed, including supine vital signs, 12-lead ECG, and collection of blood and urine specimens for clinical laboratory studies. Dual-lead ECG monitoring was performed beginning 1 h prior to dosing and continuing until 12 h postdose. A resting 12-lead ECG was also obtained at 2, 4, 8, 12, and 24 h postdose. Supine heart rate and blood pressure were obtained prior to dosing and at 0.5, 1, 2, 3, 4, 6, 8, 12, and 24 h after dosing. Assessment of adverse events was done at the same time as vital sign assessment by direct questioning, spontaneous reporting, and direct nursing observation. All subjects underwent a poststudy physical examination, including a 12-lead ECG and safety laboratory tests, approximately 7 days following the last dose of study medication.
Pharmacokinetic analyses.
Pharmacokinetic parameters were determined by fitting candidate pharmacokinetic models to the data of each subject individually, initially using the maximum likelihood procedure available in Adapt II, release 4 (4, 5). The maximum likelihood results were then used to compute maximum a posteriori Bayesian priors. To achieve the most reliable parameter estimates, the Bayesian priors also included an additional 12 subjects from another single-dose study in a similar population of healthy adult male volunteers (9). Following computation of the Bayesian priors, the data for each individual subject were reanalyzed using the maximum a posteriori fitting procedure, with the priors updated one additional time before the final parameter estimates were obtained (11). Model discrimination was accomplished using the rule of parsimony (7) and Akaike's information criterion (1). Weighting was by the fitted inverse of the residual (observation) variance; standard deviation was assumed to be linear with drug concentration. In all analyses, both enantiomers were comodeled for each individual subject. The maximum observed concentration (Cmax) and time of maximum observed concentration (Tmax) were determined by graphical inspection. Renal clearance (CLR) was computed as the total amount of each enantiomer excreted in urine over 24 h divided by the plasma area under the concentration-time curve for 24 h (AUC0–24). The standard linear trapezoidal rule was used to calculate AUC. Fractional renal clearance for each enantiomer was calculated as F × CLR/CLT, where F is oral bioavailability and CLT is total clearance. Volumes, clearances, and AUC and Cmax values were normalized for body size (65 kg); Cmax and AUC values were also normalized to dose (800 mg).
Statistical comparisons of pharmacokinetic parameters.
Related-groups analyses were accomplished using the Wilcoxon signed-rank procedure. Linear mixed effects modeling, as implemented in SAS 6.12 for Windows (8), was used for summary statistics and to test the association between dose (as a categorical independent variable) and pharmacokinetic parameter values. Because dOTC is largely cleared as unchanged drug in the urine, creatinine clearance (CCR) was considered as a covariate. Pharmacokinetic parameters tested included total oral clearances, renal clearances, steady-state apparent volumes of distribution (VSS), half-lives, Cmax, and Tmax.
RESULTS
Demographics.
Table 1 is a summary of the characteristics of the 16 male volunteers who were studied in at least one drug-containing study period. The normalized creatinine clearance (CCR in ml/min/65 kg) was estimated by the method of Cockcroft and Gault (3), based on serum creatinine, age, weight, and gender. One subject voluntarily withdrew after receiving one 800-mg dose, and another withdrew after participating in one placebo period. Both of these subjects withdrew for reasons unrelated to the study, and both completed the end-of-study evaluations and were normal. The six subjects who were studied in the 1,600-mg group, compared to those who received 200 mg, tended to be smaller (P = 0.035), younger (P = 0.062), and to have a higher normalized CCR (P = 0.098). There were no other statistically significant demographic differences between groups.
TABLE 1.
Characteristics of study volunteers
| Dose (mg)a | Age (yr) | Wt (kg) | Ht (in.) | CCR (ml/min/65 kg) |
|---|---|---|---|---|
| 100 (n = 6) | ||||
| Median | 35.5 | 72.1 | 68.6 | 83.1 |
| Mean | 35.2 | 73.3 | 69.4 | 85.4 |
| CV% | 22.0 | 14.3 | 4.70 | 17.2 |
| 200 (n = 6) | ||||
| Median | 39.6 | 80.2 | 68.6 | 78.9 |
| Mean | 38.7 | 80.1 | 69.6 | 82.0 |
| CV% | 25.5 | 5.10 | 4.40 | 12.7 |
| 400 (n = 6) | ||||
| Median | 29.3 | 76.2 | 68.6 | 90.6 |
| Mean | 30.7 | 75.1 | 69.1 | 90.6 |
| CV% | 31.6 | 4.80 | 3.00 | 20.0 |
| 800 (n = 7) | ||||
| Median | 33.6 | 77.3 | 69.1 | 85.7 |
| Mean | 33.3 | 75.3 | 68.6 | 82.4 |
| CV% | 38.2 | 6.40 | 1.90 | 19.1 |
| 1,600 (n = 6) | ||||
| Median | 23.2 | 73.4 | 67.6 | 90.8 |
| Mean | 26.9 | 71.1 | 68.5 | 91.9 |
| CV% | 28.6 | 10.0 | 3.80 | 9.30 |
| All data (n = 16) | ||||
| Median | 34.7 | 76.2 | 68.6 | 86.1 |
| Mean | 33.2 | 75.1 | 69.0 | 86.9 |
| CV% | 30.4 | 8.98 | 3.47 | 17.3 |
n, number of participants.
Safety and tolerability.
All oral doses were well tolerated by the subjects, and no serious adverse events were reported during the study. A total of 11 adverse events were reported by eight subjects, the most common being headache, upper respiratory tract infection, and irritation and redness at the intravenous catheter site, each being reported twice. None of these events were considered to be related to the study drug, and all were resolved prior to completion of the study. There were no clinically significant changes in ECGs or laboratory values, and all vital signs remained within normal limits. No abnormalities were reported following the end-of-study physical examinations.
Pharmacokinetics.
Log-linear plots of individual concentration-time profiles showed that there was no clear, terminal, log-linear phase for a study of 24 h duration, with the apparent half-lives based on the last three observations being systematically shorter than half-lives based on the last two observations. Therefore, the sampling strategy was altered for the 800- and 1,600-mg study periods; the 6- and 10-h samples were omitted, and samples at 36 and 48 h were added.
The final pharmacokinetic model was a linear, two-compartment model, with absorption occurring during one to three first-order input phases, each following a fitted lag time (Fig. 1). Another study using intravenous dOTC determined that a three-compartment model was superior, by Akaiki's Information Criterion, to a two-compartment model (9). However, when dOTC is administered orally, absorption proceeds simultaneously with distribution, masking the ability to distinguish between the distributional compartments. Therefore, a two-compartment model is utilized for oral dOTC. Details of the three-compartment pharmacokinetic model have been reported previously (9).
FIG. 1.
Two-compartment pharmacokinetic model for (−)dOTC and (+)dOTC.
All subjects were fitted assuming that absorption characteristics for the two enantiomers were similar. The percent of total dose absorbed (D%) in phases I and III were fitted parameters, enabling direct computation of the phase II D%, as the total sum of D%'s was required to equal 100%. The fit of the model to the data was excellent: for (+)dOTC, the median r2 was 0.998, with a range of 0.993 to 1.00; for (−)dOTC, the median r2 was 0.998, with a range of 0.992 to 1.00. The mean plasma concentration versus time curves for (−)dOTC and (+)dOTC are presented in Fig. 2. The final pharmacokinetic parameters for each enantiomer are summarized in Tables 2 and 3; absorption characteristics are summarized in Table 4.
FIG. 2.
Mean (−)dOTC (A) and (+)dOTC (B) concentration versus time plots for all subjects following oral doses of racemic dOTC.
TABLE 2.
Pharmacokinetic parameters for oral (−)dOTCa
| Dose (mg) | VSS (liter/65 kg) | CLD (liter/h/65 kg) | CLT/F (liter/h/65 kg) | CLR (liter/h/65 kg) | Cmax (μg/ml/65 kg) | Tmax (h) | λZt1/2 (h) |
|---|---|---|---|---|---|---|---|
| 100 (n = 6) | |||||||
| Median | 28.4 | 0.808 | 19.0 | 13.1 | 0.55 | 2.0 | 14.3 |
| Mean | 29.8 | 1.17 | 18.6 | 13.1 | 0.60 | 2.1 | 14.5 |
| CV% | 13.2 | 73.8 | 17.3 | 14.4 | 41.0 | 61.0 | 53.3 |
| 200 (n = 6) | |||||||
| Median | 30.1 | 1.41 | 20.4 | 12.5 | 0.93 | 1.0 | 8.8 |
| Mean | 29.1 | 1.24 | 20.4 | 12.6 | 1.00 | 1.2 | 10.2 |
| CV% | 10.0 | 30.9 | 15.7 | 19.9 | 25.3 | 42.1 | 29.4 |
| 400 (n = 6) | |||||||
| Median | 36.1 | 1.18 | 20.6 | 15.5 | 1.43 | 1.25 | 11.1 |
| Mean | 37.6 | 1.42 | 21.2 | 14.8 | 1.66 | 1.67 | 11.8 |
| CV% | 18.5 | 53.5 | 16.4 | 21.2 | 35.7 | 64.1 | 35.6 |
| 800 (n = 7) | |||||||
| Median | 29.3 | 1.53 | 19.0 | 13.6 | 2.9 | 3.0 | 9.3 |
| Mean | 34.0 | 1.35 | 21.3 | 13.1 | 2.9 | 2.5 | 10.3 |
| CV% | 40.8 | 60.5 | 21.8 | 25.5 | 34.4 | 51.0 | 39.3 |
| 1,600 (n = 6) | |||||||
| Median | 33.0 | 1.45 | 23.3 | 13.9 | 6.0 | 1.0 | 8.5 |
| Mean | 40.1 | 2.25 | 26.1 | 14.3 | 5.9 | 1.4 | 10.1 |
| CV% | 46.7 | 101.0 | 31.5 | 19.6 | 39.2 | 63.8 | 42.3 |
| All data (n = 16) | |||||||
| Median | 30.1 | 1.39 | 20.3 | 13.6 | 1.5 | 9.5 | |
| Mean | 34.1 | 1.48 | 21.5 | 13.6 | NA | 1.8 | 11.3 |
| CV% | 33.1 | 79.8 | 24.3 | 20.2 | 61.6 | 42.7 |
CLD, distributional clearance between the central and peripheral compartments; λ, t1/2, terminal half-life; n, number of participants; NA, not applicable.
TABLE 3.
Pharmacokinetic parameters for oral (+)dOTCa
| Dose (mg) | VSS (liter/65 kg) | CLD (liter/h/65 kg) | CLT/F (liter/h/65 kg) | CLR (liter/h/65 kg) | Cmax (μg/ml/65 kg) | Tmax (h) | λZt1/2 (h) |
|---|---|---|---|---|---|---|---|
| 100 (n = 6) | |||||||
| Median | 58.7 | 2.29 | 15.0 | 9.76 | 0.66 | 2.0 | 17.4 |
| Mean | 56.1 | 2.35 | 14.7 | 10.1 | 0.71 | 1.9 | 16.6 |
| CV% | 16.9 | 25.9 | 16.1 | 13.1 | 36.1 | 62.1 | 32.4 |
| 200 (n = 6) | |||||||
| Median | 60.2 | 2.52 | 16.8 | 10.0 | 1.13 | 1.0 | 16.1 |
| Mean | 59.9 | 2.52 | 16.3 | 9.8 | 1.23 | 1.08 | 15.9 |
| CV% | 7.20 | 14.0 | 13.2 | 18.7 | 29.1 | 31.5 | 13.9 |
| 400 (n = 6) | |||||||
| Median | 68.3 | 2.88 | 16.4 | 12.2 | 1.53 | 1.25 | 14.5 |
| Mean | 68.1 | 2.98 | 17.8 | 11.3 | 1.85 | 1.67 | 15.0 |
| CV% | 11.0 | 33.1 | 21.6 | 19.7 | 44.2 | 64.1 | 20.7 |
| 800 (n = 7) | |||||||
| Median | 52.6 | 2.70 | 16.0 | 10.5 | 3.15 | 1.5 | 13.7 |
| Mean | 58.8 | 2.61 | 17.4 | 9.97 | 3.33 | 1.9 | 14.8 |
| CV% | 27.8 | 42.7 | 21.5 | 24.8 | 32.8 | 59.0 | 29.5 |
| 1,600 (n = 6) | |||||||
| Median | 61.7 | 2.88 | 19.4 | 10.1 | 7.37 | 0.88 | 11.8 |
| Mean | 66.5 | 3.51 | 21.1 | 10.8 | 7.30 | 1.04 | 14.3 |
| CV% | 38.4 | 67.9 | 32.6 | 19.8 | 40.1 | 35.3 | 43.2 |
| All data (n = 16) | |||||||
| Median | 60.2 | 2.63 | 16.3 | 10.3 | 1.5 | 14.8 | |
| Mean | 61.8 | 2.79 | 17.5 | 10.4 | NA | 1.5 | 15.3 |
| CV% | 23.5 | 45.5 | 25.1 | 19.3 | 61.3 | 27.8 |
CLD, distributional clearance between the central and peripheral compartments; λZt1/2, terminal half-life; n, number of participants; NA, not applicable.
TABLE 4.
Absorption characteristics of (+)dOTC and (−)dOTC for the two-compartment pharmacokinetic modela
| Dose (mg) | Tlag 1 (h) | ka 1 (h−1) | D% 1 | Tlag 2 (h) | ka 2 (h−1) | D% 2 | Tlag 3 (h) | ka 3 (h−1) | D% 3 |
|---|---|---|---|---|---|---|---|---|---|
| 100 (n = 6) | |||||||||
| Median | 0.382 | 1.16 | 21.8 | 1.23 | 0.700 | 39.1 | 3.00 | 0.312 | 52.6 |
| Mean | 0.367 | 1.96 | 23.7 | 1.13 | 0.810 | 41.7 | 2.83 | 0.316 | 51.9 |
| CV% | 21.4 | 92.9 | 69.0 | 21.9 | 63.5 | 38.5 | 24.6 | 14.4 | 20.8 |
| 200 (n = 6) | |||||||||
| Median | 0.335 | 0.655 | 29.4 | 0.578 | 5.72 | 23.2 | 2.48 | 0.265 | 38.5 |
| Mean | 0.349 | 0.953 | 31.9 | 0.685 | 4.65 | 32.6 | 2.16 | 0.281 | 42.7 |
| CV% | 36.2 | 106 | 49.8 | 40.5 | 73.3 | 71.6 | 73.3 | 19.8 | 32.6 |
| 400 (n = 6) | |||||||||
| Median | 0.418 | 2.33 | 35.4 | 0.989 | 1.54 | 23.4 | 1.96 | 0.330 | 44.3 |
| Mean | 0.369 | 2.61 | 35.4 | 1.10 | 1.86 | 27.6 | 2.83 | 0.401 | 44.4 |
| CV% | 33.1 | 89.0 | 33.6 | 58.3 | 82.5 | 76.5 | 58.5 | 53.1 | 20.9 |
| 800 (n = 7) | |||||||||
| Median | 0.248 | 1.72 | 24.4 | 1.21 | 0.274 | 44.4 | 2.00 | 0.368 | 37.1 |
| Mean | 0.329 | 2.54 | 30.1 | 1.11 | 1.21 | 43.4 | 2.59 | 0.456 | 37.0 |
| CV% | 36.6 | 109 | 79.3 | 47.2 | 153 | 54.0 | 39.1 | 43.8 | 35.3 |
| 1,600 (n = 6) | |||||||||
| Median | 0.246 | 1.03 | 36.5 | 68.9 | 0.277 | 33.4 | 2.19 | 0.476 | 29.9 |
| Mean | 0.314 | 2.19 | 32.0 | 79.4 | 1.71 | 36.7 | 2.22 | 0.897 | 31.3 |
| CV% | 38.0 | 104 | 56.5 | 40.8 | 40.8 | 67.8 | 52.1 | 130 | 49.9 |
| All data (n = 16) | |||||||||
| Median | 0.356 | 0.913 | 30.9 | 0.911 | 0.694 | 33.1 | 2.39 | 0.316 | 39.9 |
| Mean | 0.345 | 2.07 | 30.6 | 0.970 | 2.02 | 36.6 | 2.50 | 0.493 | 40.6 |
| CV% | 31.6 | 101 | 56.0 | 46.2 | 127 | 58.3 | 42.7 | 121 | 33.9 |
Tlag, lag time for oral absorption; ka, rate constant for oral absorption; n, number of participants.
Comparing enantiomers, median modeled values of oral clearance for (+)dOTC were 19.5% lower than those for (−)dOTC (P < 0.001), the oral total steady-state distribution volumes were 100% larger (P < 0.001), and the terminal plasma half-life was 55.2% longer (P < 0.001). The intersubject variability in total oral clearance (CLT/F), for the two-compartment model, was quite small [the CV for (+)dOTC was 17.4% and for (−)dOTC was 18.7%]. The median and mean values of Cmax (normalized for body size) were larger for (+)dOTC than for (−)dOTC (P = 0.002). There was no significant difference in Tmax (P > 0.6) between enantiomers.
When CLR values were compared between enantiomers, (+)dOTC was 23.5% lower than (−)dOTC (P < 0.05), which is consistent with the difference found in total oral clearances. The intersubject variability of CLR was also low, with a CV less than or equal to 20% for both enantiomers. The median CV fraction of total clearance that was renal for (−)dOTC and (+)dOTC was 0.66 (23.9%) and 0.62 (22.8%), respectively. Because CLT/F is conditioned on F and CLR is not, the fractional renal clearance (F × CLR/CLT) should be adjusted for absolute bioavailability, which was determined to be approximately 80% for both (−)dOTC and (+)dOTC in a separate study (9). When bioavailability is considered, it suggests that 80 to 90% of bioavailable dOTC is eliminated unchanged in the urine. Lastly, because the values of renal clearance for (−)dOTC and (+)dOTC are much larger than normal values of the glomerular filtration rate, it appears that both enantiomers undergo active secretion into the urine.
AUC0–24 for both enantiomers was observed to increase linearly with increasing doses across all subjects. A significant difference by dose was found between oral clearances of both (−)dOTC (P = 0.023) and (+)dOTC (P = 0.023) between the 100- and 1,600-mg doses. Upon further inspection, it became clear that the difference in the 1,600-mg group was due to a single subject outlier. This subject's oral clearance of both isomers in the 1,600-mg dose [40.6 and 33.7 liter/h/65 kg for (−)dOTC and (+)dOTC, respectively] was significantly higher than that of the other subjects at the same dose level and was also significantly higher than the same subject's oral clearance when administered 800 mg [25.2 and 20.0 liter/h/65 kg for (−)dOTC and (+)dOTC, respectively]. This subject's renal clearances in the two study periods were not different, indicating strongly that the observed difference was due to a decreased oral absorption during the 1,600-mg study period. When this subject was removed from the analysis, there was no significant difference in oral clearance. Figures 3 and 4 depict the lack of effect of dose on total oral and renal clearances, with a dashed line identifying the single subject outlier in oral clearance. No other tested pharmacokinetic parameters differed significantly by dose (P > 0.05 for all comparisons).
FIG. 3.
Comparison of total oral clearances and dose across all subjects for (−)dOTC and (+)dOTC. Lines connect study periods for each individual subject; the dashed line indicates the individual subject outlier in oral clearance.
FIG. 4.
Comparison of renal clearances and dose across all subjects for (−)dOTC and (+)dOTC. Lines connect study periods for each individual subject; the dashed line indicates the individual subject outlier in oral clearance.
DISCUSSION
The results of the present study found racemic dOTC to be safe and well tolerated following single oral doses between 100 and 1,600 mg. Plasma pharmacokinetics of oral (+)dOTC and (−)dOTC are well described by a linear, two-compartment model (in addition to the absorption site) with elimination from the central compartment. The data showed multiple peaks and other changes in slope after oral administration. This behavior was well described by two or three “absorption phases,” in which a portion of the dose is released and absorbed, with each phase having a separate lag time and absorption rate constant.
The pharmacokinetics of dOTC showed small intersubject variability. When (+)dOTC was compared to (−)dOTC, median CLT/F was 19.5% lower, terminal half-life was 55.2% longer, and Vss/F was 100% larger for (+)dOTC. There was no significant effect of dose size on the pharmacokinetics of dOTC. The relatively long half-lives would support dosing intervals of 12 to 24 h. However, as with all nucleoside reverse transcriptase inhibitors, a long plasma half-life may not equate to a long duration of drug action, as intracellular triphosphate concentrations are the active moiety. Early clinical efficacy data from short-term monotherapy studies have found racemic dOTC to have significant anti-HIV-1 activity when administered either once or twice daily (R. Wood, B. Trope, R. Van Leeuwen, D. E. Martin, and L. Proulx, Abstr. 39th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 503, 1999), and efficacy was not related to plasma trough concentrations (10).
An interim analysis found a longer half-life than expected, making it necessary to alter the sampling strategy midway through the study. The duration of sampling was therefore extended from 24 to 48 h for the 800- and 1,600-mg doses. This change allowed an improved characterization of the terminal elimination phase, which may not have been adequately described with only 24 h of serum sampling. The change also provides an excellent example of the utility of having real-time access to interim study results to allow modification of sampling strategies early in phase I investigations.
The primary route of dOTC elimination is renal (80 to 90% of bioavailable drug is eliminated unchanged in the urine) and must include active secretion of both enantiomers. This large degree of renal elimination may require a dose adjustment in patients suffering from renal impairment. This dependence on renal clearance also makes drug interactions involving hepatic enzymes less likely; however, interactions with renally secreted drugs may be present and should be investigated in future studies.
Due to the limited number of agents currently available to treat HIV disease, more effective medications are needed as alternatives to both empiric and salvage therapy. Agents with long half-lives that can be dosed infrequently and thereby improve adherence are also desirable. Racemic dOTC is a mixture of two enantiomers that has shown activity against HIV-1 strains of virus resistant to other reverse transcriptase inhibitors and has a beneficial pharmacokinetic profile. Future clinical trials will be needed to establish the safety, tolerability, and efficacy of chronic administration in the treatment of HIV infection.
ACKNOWLEDGMENTS
This work was supported in part by a grant from BioChem Pharma Inc.
We thank John Adams for his insightful comments regarding the content of the manuscript.
REFERENCES
- 1.Akaike H. A Bayesian extension of the minimum AIC procedure of autoregressive model fitting. Biometrika. 1979;66:237–242. [Google Scholar]
- 2.Carpenter C C, Cooper D A, Fischl M A, Gatell J M, Gazzard B G, Hammer S M, et al. Antiretroviral therapy in adults: updated recommendations of the International AIDS Society-USA Panel. JAMA. 2000;283:381–390. doi: 10.1001/jama.283.3.381. [DOI] [PubMed] [Google Scholar]
- 3.Cockcroft D W, Gault M H. Prediction of creatinine clearance from serum creatinine. Nephron. 1976;16:31–41. doi: 10.1159/000180580. [DOI] [PubMed] [Google Scholar]
- 4.D'Argenio D Z, Schumitzky A. A program package for simulation and parameter estimation in pharmacokinetic systems. Comput Programs Biomed. 1979;9:115–134. doi: 10.1016/0010-468x(79)90025-4. [DOI] [PubMed] [Google Scholar]
- 5.D'Argenio D Z, Schumitzky A. Adapt II users guide. Biomedical simulations resource. Los Angeles, Calif: University of Southern California; 1999. [Google Scholar]
- 6.De Muys J-M, Gourdeau H, Nguyen-Ba N, Taylor D L, Ahmed P S, Mansour T, Locas C, Richard N, Wainberg M A, Rando R F. Anti-human immunodeficiency virus type 1 activity, intracellular metabolism, and pharmacokinetic evaluation of 2′-deoxy-3′-oxa-4′-thiocytidine. Antimicrob Agents Chemother. 1999;43:1835–1844. doi: 10.1128/aac.43.8.1835. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Jusko W J. In W. E. Evans, W. J. Jusko, and J. J. Schentag (ed.), Applied pharmacokinetics: principles of therapeutic drug monitoring. 3rd ed. Vancouver, Wash: Applied Therapeutics, Inc.; 1992. Guidelines for collection and analysis of pharmacokinetic data, p. 2:1–43. [Google Scholar]
- 8.Littell J, Ramon C, Milliken G A, Stroup W W, Wolfinger R D. SAS System for Mixed Models. Cary, N.C: SAS Institute, Inc.; 1996. [Google Scholar]
- 9.Smith P F, Forrest A, Ballow C H, Martin D E, Proulx L. Absolute bioavailability and disposition of (−) and (+) 2′-deoxy-3′-oxa-4′-thiocytidine (dOTC) following single intravenous and oral doses of racemic dOTC in humans. Antimicrob Agents Chemother. 2000;44:1609–1615. doi: 10.1128/aac.44.6.1609-1615.2000. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Smith P F, Forrest A, Rayner C, Wood R, Proulx L. Integrating viral dynamics and pharmacokinetics of racemic dOTC in HIV-infected antiretroviral naive patients, abstr. WePe4119. Durban, South Africa: XIII International AIDS Conference; 2000. [Google Scholar]
- 11.Steimer J L, Mallet A, Golmard J L, Boisvieux J F. Alternative approaches to estimation of population pharmacokinetic parameters: comparison with the nonlinear mixed-effect model. Drug Metab Rev. 1984;15:265–292. doi: 10.3109/03602538409015066. [DOI] [PubMed] [Google Scholar]