Evaluation of the GastroPlus™ Advanced Compartmental and Transit (ACAT) Model in Early Discovery.

PURPOSE: The aim of this study was to evaluate the oral exposure predictions obtained early in drug discovery with a generic GastroPlus Advanced Compartmental And Transit (ACAT) model based on the in vivo intravenous blood concentration-time profile, in silico properties (lipophilicity, pKa) and in vitro high-throughput absorption-distribution-metabolism-excretion (ADME) data (as determined by PAMPA, solubility, liver microsomal stability assays). METHODS: The model was applied to a total of 623 discovery molecules and their oral exposure was predicted in rats and/or dogs. The predictions of Cmax, AUClast and Tmax were compared against the observations. RESULTS: The generic model proved to make predictions of oral Cmax, AUClast and Tmax within 3-fold of the observations for rats in respectively 65%, 68% and 57% of the 537 cases. For dogs, it was respectively 77%, 79% and 85% of the 124 cases. Statistically, the model was most successful at predicting oral exposure of Biopharmaceutical Classification System (BCS) class 1 compounds compared to classes 2 and 3, and was worst at predicting class 4 compounds oral exposure. CONCLUSION: The generic GastroPlus ACAT model provided reasonable predictions especially for BCS class 1 compounds. For compounds of other classes, the model may be refined by obtaining more information on solubility and permeability in secondary assays. This increases confidence that such a model can be used in discovery projects to understand the parameters limiting absorption and extrapolate predictions across species. Also, when predictions disagree with the observations, the model can be updated to test hypotheses and understand oral absorption.

Structure-based design, synthesis, and biological evaluation of irreversible human rhinovirus 3C protease inhibitors. 4. Incorporation of P1 lactam moieties as L-glutamine replacements.

The structure-based design, chemical synthesis, and biological evaluation of various human rhinovirus (HRV) 3C protease (3CP) inhibitors which incorporate P1 lactam moieties in lieu of an L-glutamine residue are described. These compounds are comprised of a tripeptidyl or peptidomimetic binding determinant and an ethyl propenoate Michael acceptor moiety which forms an irreversible covalent adduct with the active site cysteine residue of the 3C enzyme. The P1-lactam-containing inhibitors display significantly increased 3CP inhibition activity along with improved antirhinoviral properties relative to corresponding L-glutamine-derived molecules. In addition, several lactam-containing compounds exhibit excellent selectivity for HRV 3CP over several other serine and cysteine proteases and are not appreciably degraded by a variety of biological agents. One of the most potent inhibitors (AG7088, mean antirhinoviral EC90 approximately 0.10 microM, n = 46 serotypes) is shown to warrant additional preclinical development to explore its potential for use as an antirhinoviral agent.

In vitro models to predict the in vivo mechanism, rate, and extent of placental transfer of dideoxynucleoside drugs against human immunodeficiency virus.

OBJECTIVE: We tested the hypothesis that the mechanism, rate, and extent of in vivo placental transfer of dideoxynucleoside drugs against human immunodeficiency virus can be predicted by the in vitro perfused human placenta and the drug octanol-water partition coefficient. STUDY DESIGN: Near-term pregnant macaques (Macaca nemestrina ) underwent long-term catheterization for the administration of 4 dideoxynucleosides against human immunodeficiency virus: zidovudine, didanosine, zalcitabine, and stavudine. Maternal plasma, fetal plasma, and amniotic fluid concentrations were determined frequently after intravenous bolus and/or infusion of the drugs administered into the maternal or fetal circulation on separate occasions. Antipyrine was included in all experiments as a marker of placental blood flow. The mechanism, rate, and extent of placental transfer of the 4 dideoxynucleosides in perfused human placenta were determined and compared with the findings obtained by others. RESULTS: The mechanism and rate of the antipyrine-normalized placental transfer of the 4 dideoxynucleosides in perfused human placenta were highly correlated with those observed in vivo. The extent of placental transfer (fetal/maternal steady-state plasma concentration ratio) was also highly correlated with both the antipyrine-normalized placental transfer clearance (clearance index) determined in the in vitro perfused human placenta model (r 2 = 0.95, in vitro clearance-index model) and the drug octanol-water partition coefficient (r 2 = 0.99, in vitro partition-coefficient model). To determine the predictive capacity of these correlative models, we predicted the fetal/maternal steady-state plasma concentration ratio of each drug after excluding the data on that drug from the model fit. Both in vitro models to predict in vivo placental transfer of drug models resulted in good predictions of the observed fetal/maternal steady-state plasma concentration ratio (mean error: in vitro clearance-index model = -1. 2%; in vitro partition-coefficient model = 3.9%). CONCLUSIONS: We propose that our models will accurately predict the extent of placental transfer of dideoxynucleoside drugs against human immunodeficiency virus. The models may also be applicable to other classes of drugs, regardless of therapeutic category, provided that these drugs passively diffuse across the placenta. Such a result will expedite phase 1 clinical trials of drugs in pregnant women.

In vivo maternal-fetal-amniotic fluid pharmacokinetics of zidovudine in the pigtailed macaque: comparison of steady-state and single-dose regimens.

The purpose of this study was to characterize the disposition of zidovudine in the maternal-fetal-amniotic fluid unit in vivo. Zidovudine was administered as a constant-rate infusion or a bolus dose to the dam, fetus and amniotic cavity (bolus dose only) of the near-term pigtailed macaque model. A fetal-maternal plasma steady-state concentration ratio of 0.76 +/- 0.06 suggested that the drug was transferred extensively to the fetal compartment. Similarly, the mean fetal-maternal plasma area under the curve (AUC) ratio after administration of an i.v. bolus dose to the dam was 0.84 +/- 0. 09. Both ratios were significantly less than unity (P < .05), which indicates that fetal exposure to zidovudine was lower than maternal exposure. The placental transfer of zidovudine was passive, with a clearance of approximately 2.0 ml/min/kg, about 35% of the rate of the placental blood flow marker antipyrine. Zidovudine concentration in the amniotic fluid was higher than that in the fetal plasma because the drug is eliminated slowly from the amniotic cavity. The steady-state and i.v. bolus experimental designs resulted in close estimates of the extent of placental transfer of zidovudine (steady-state fetal-maternal plasma concentration ratio or fetal-maternal plasma AUC ratio), which indicates that the extent of transfer of zidovudine is independent of the mode of drug administration. We predict that when zidovudine is administered orally to pregnant women, the average fetal exposure to zidovudine will be approximately three fourths of the maternal exposure. This observation suggests that the dose administered to the pregnant woman need not be changed even if the fetus is the primary target of therapy.

Effect of pregnancy, mode of administration and neonatal age on the pharmacokinetics of zalcitabine (2', 3'-dideoxycytidine) in the pigtailed macaque (Macaca nemestrina).

Our objective was to determine the effect of pregnancy, mode of administration and neonatal age on the pharmacokinetics of the anti-HIV drug zalcitabine (2',3'-dideoxycytidine; ddC) in the pigtailed macaque (Macaca nemestrina). Zalcitabine was administered as an i.v. bolus dose to pregnant dams (n = 3) at term and at 6 weeks post-partum. No significant differences were found between the pre- and post-partum systemic plasma clearance, steady-state volume of distribution or terminal plasma half-life of zalcitabine, indicating that pregnancy does not affect the pharmacokinetics of the drug in the macaque. The observed maternal plasma, fetal plasma and amniotic fluid concentration-time profiles were compared with profiles that were simulated using pharmacokinetic parameter estimates obtained in an earlier constant i.v. infusion study in pregnant macaques. The fetal:maternal ratio of the area under the simulated zalcitabine plasma concentration-time profile after an i.v. bolus dose (0.58) was close to the earlier observed fetal:maternal steady-state plasma concentration ratio after i.v. infusion of the drug (0.58 +/- 0.05). The excellent agreement between observed and simulated fetal:maternal ratio of zalcitabine demonstrates that the steady-state infusion experimental design can be used to estimate the drug exposure to the fetus after a single dose. To determine the influence of age on the pharmacokinetics of zalcitabine, the drug was administered as a single i.v. bolus dose to four infant macaques serially at the ages of 1-2 weeks, 1 month and 4 months. The systemic plasma clearance of zalcitabine was significantly smaller and the terminal plasma half-life significantly longer at age 1-2 weeks than at 1 and 4 months of age. If replicated in humans, these substantial age-dependent changes in the pharmacokinetics of zalcitabine would warrant smaller and less frequent dosing with zalcitabine in HIV-infected neonates than in older children and adults.

Mechanism and rate of placental transfer of zalcitabine (2',3'-dideoxycytidine) in Macaca nemestrina.

OBJECTIVE: Our purpose was to determine whether the ant-human immunodeficiency virus drug zalcitabine (2',3'-dideoxycytidine) is actively transferred across the placenta in the near-term Macaca nemestrina. STUDY DESIGN: Constant rate infusions of zalcitabine (1.31 microg/min/kg) and antipyrine (66.7 microg/min/kg) were administered to the dam through the femoral vein (n = 4) or to the fetus through the carotid artery (n = 3) in a randomized cross-over design. Zalcitabine was also administred at a 10-fold higher infusion rate to the dam (n = 3). The effect of zidovudine on the transplacental transfer of zalcitabine was studied by coinfusing the two drugs to the dam (n = 2). Samples from maternal plasma, fetal plasma, and amniotic fluid were collected at regular intervals during the 30-hour infusions. RESULTS: The maternal-fetal transfer clearance of zalcitabine (0.41 +/- 0.16 ml/min/kg) was not significantly different from the fetal-maternal transfer clearance of the drug (0.66 +/ 0.30 ml/min/kg). Moreover, the steady-state fetal-maternal plasma concentration ratios of zalcitabine after the low 0.58 +/- 0.06) and high (0.66 +/- 0.10) infusion rates to the dam were similar. This ratio was not substantially changed (0.69) when zalcitabine was coadministered with zidovudine. The placental transfer rate of zalcitabine was 11% (+/-4%) that of antipyrine. CONCLUSION: The placental transfer of zalcitabine is passive and unaffected by simultaneous administration of zidovudine. In Macaca nemestrina the average fetal exposure to zalcitabine is approximately 60% of the maternal exposure.

Efflux of zidovudine and 2',3'-dideoxyinosine out of the cerebrospinal fluid when administered alone and in combination to Macaca nemestrina.

To determine if there is active efflux of zidovudine (ZDV) and 2',3'-dideoxyinosine (ddI) out of the cerebrospinal fluid (CSF), and if this efflux is saturable, we investigated the steady-state CSF/plasma concentration ratio of the two drugs when administered alone or in combination. Constant-rate infusions of ZDV, ddI or both were administered to seven macaques (Macaca nemestrina) through a chronic venous catheter for a minimum of 28 hr. Antipyrine, a marker of passive diffusion, was coinfused in all experiments. Blood (5 mL) and CSF samples (0.5-1 mL) were collected by venous and lumbar/thoracic punctures, respectively, at 24 and 28 hr after beginning the infusion. When ZDV and ddI were administered alone, the steady-state CSF/plasma concentration ratios were significantly different from unity (ZDV, 0.20 +/- 0.08; ddI, 0.09 +/- 0.04) and were independent of the plasma concentration (P > 0.05). In contrast, the CSF/plasma concentration ratio of antipyrine (0.82 +/- 0.19) was close but significantly smaller than unity (P > 0.05). The CSF/plasma concentration ratios after simultaneous administration of ZDV and ddI were not significantly different (P > 0.05) from those obtained after administration of the drugs alone. These results suggest that ZDV and ddI are actively transported out of the CSF; however, within the concentration range studied, this efflux is neither saturable nor mutually competitive. Concomitant administration of ZDV and ddI did not produce a systemic interaction in the animals, indicating that the pharmacokinetics of either drug is unaffected by the presence of the other.

No effect of vitamin B-6 against premenstrual tension. A controlled clinical study.

Vitamin B-6 100 mg given daily throughout the menstrual cycle was compared with placebo in a randomized, double-blind crossover trial in 34 women who suffered from premenstrual tension. Vitamin B-6 was no better than placebo. There was a substantial period effect, as the women evidenced a considerable preference for the second drug they received, irrespective of whether this was vitamin B-6 or placebo. Blood magnesium was measured; no significant difference was found between the 34 women with premenstrual tension and 10 healthy women without such complaints. Vitamin B-6 caused a small but statistically significant rise in blood magnesium level. In the individual patients, no correlation was found between changes in blood magnesium and premenstrual symptoms.