William Hageman

Allometric scaling of pharmacokinetic parameters in drug discovery: Can human CL, Vss and t1/2 be predicted fromin-vivo rat data?

SummaryIn a drug discovery environment, reasonable go/no-go humanin-vivo pharmacokinetic (PK) decisions must be made in a timely manner with a minimum amount of animalin-vivo orin-vitro data. We have investigated the accuracy of thein-vivo correlation between rat and human for the prediction of the total systemic clearance (CL), the volume of distribution at steady state (Vss), and the half-life (t1/2) using simple allometric scaling techniques. We have shown, using a large diverse set of drugs, that a fixed exponent allometric scaling approach can be used to predict humanin-vivo PK parameters CL, Vss and t1/2 solely from ratin-vivo PK data with acceptable accuracy for making go/no-go decisions in drug discovery. Humanin-vivo PK predictions can be obtained using the simple allometric scaling relationships CLHuman ≈ 40 CLRat (L/hr), Vss Human ≈ 200 Vss Rat (L), and t1/2 Human ≈ 4 t1/2 Rat (hr). The average fold error for human CL predictions for N=176 drugs was 2.25 with 79% of the drugs having a fold error less than 3. The average fold error for human Vss predictions for N=144 drugs was 1.85 with 84% of the drugs having a fold error less than 3. The average fold error for human t1/2 predictions for N=145 drugs was 2.05 with 76% of the drugs having a fold error less than 3. Using these simple allometric relationships, the sorting of drug candidates into a low/medium/high/very high human classification scheme was also possible from rat data. Since these simple allometric relationships between rat and human CL, Vss, and t1/2 are reasonably accurate, easy to remember and simple to calculate, these equations should be useful for making early go/no-goin-vivo human PK decisions for drug discovery candidates.

Improved pharmacokinetic and bioavailability support of drug discovery using serial blood sampling in mice

Pharmacokinetic studies in mice traditionally require one animal per time point, resulting in dosing and euthanizing a large number of animals and producing suboptimal quality of pharmacokinetic data due to inter-animal variability and dosing error. These studies are time-consuming and labor-intensive. To improve the throughput and quality of pharmacokinetic evaluation in mice, we have developed a serial blood sampling methodology using the lateral saphenous vein puncture technique. Two marketed drugs, indinavir and rosuvastatin, were selected for this validation study because of their distinctly different physicochemical and pharmacokinetic properties. Each compound was dosed orally and intravenously in mice using both discrete and serial blood sampling methods. The pharmacokinetic results from serial bleeding are in excellent agreement with those from discrete sampling for both compounds. Compared to the discrete sampling, the serial sampling procedure is a more humane method, allowing for rapid and repeated sampling from the same site without the need for anesthesia. The application of this new method has led to a remarkable reduction in animal and compound usage, a significant increase in throughput and speed, and a drastic improvement in pharmacokinetic data quality. This approach is especially useful for the first-tier in vivo pharmacokinetic screening of discovery compounds.

Synthesis and biological evaluation of novel indoloazepine derivatives as non-peptide vasopressin V2 receptor antagonists.

A series of novel 3,4,5,6-tetrahydro-1H-azepino[4,3,2-cd]indoles was synthesized and tested for vasopressin receptor antagonist activity. We identified compounds with high affinity for the human V2 receptor and good selectivity over the human V1a receptor. Compound 6c bound to V2 receptors with an IC(50) value of 20 nM, had >100-fold selectivity over V1a receptors, and inhibited cAMP formation in a cellular V2 functional assay with an IC(50) value of 70 nM.

Bridged bicyclic vasopressin receptor antagonists with V2-Selective or dual V1a/V2 activity

The synthesis and biological testing of a novel series of nonpeptide vasopressin receptor antagonists, containing a bridged bicyclic nucleus, are reported. Variation of substituents (R(1)-R(3)) in general formula 3, and the configuration of the stereocenter, resulted in potent V(2)-selective (e.g., 5) and balanced dual V(1a)/V(2) (e.g., 10) compounds. Data from receptor binding, cell-based functional, and in vivo assays are presented [corrected]

The use of the suicide CYP450 inhibitor ABT for distinguishing absorption and metabolism processes in in-vivo pharmacokinetic screens

SummarySince drug candidates with low oral systemic exposure may be due to either or both absorption and metabolism factors, determining what factors limit the oral systemic exposure is not always obvious in a singlein-vivo pharmacokinetic (PK) assay. A rapid ratin-vivo PK screen where the oxidative drug metabolism has been attenuated using the suicide CYP450 inhibitor aminobenzotriazole (ABT) is described. We have shown that the roles of absorption and metabolism for drug candidates with low oral systemic exposure can be determined by comparing the PK parameters of drug candidates orally administered to non-treated and ABT-treated rats. Propranolol, metoprolol and climetidine are used as model drugs. Propranolol and metoprolol have low oral systemic exposures in rats primarily due to metabolism factors while the oral systemic exposure of climetidine is high in rats. For propranolol and metoprolol, large increases in the systemic exposure of these drugs were observed between non-treated and ABT-treated rats. ABT appeared not to increase or decrease significantly the rate and extent of absorption or metabolism of Cimetidine since that oral systemic exposure of non-treated and ABT-treated rats did not significantly change.These experiments suggest that for drug candidates with low systemic exposures in rats an observation of no change in the oral systemic exposure in ABT-treated rats when compared to the non-treated rats imply that absorption (or formulation) factors limit the systemic exposure of the drug while an increase in the systemic exposure in ABT-treated rats imply that metabolism factors limit the systemic exposure. Due to the ease of preparing and interpreting PK data from ABT-treated rats, is suggested that this assay could be used as an alternative toin vivo cannulation assays. Please send reprint requests to: Dr G. W. Caldwell,

Applied Pharmacokinetics in Drug Development

The process of discovering, developing, and marketing new drugs has changed considerably in the last decade; however, the cost associated with this process remains staggeringly high. Although there are many reasons for this high cost, one reason appears to be the continuing high attrition rates of drugs during costly early- and late-stage human clinical trials. To address this problem, drug discovery organizations are striving to rapidly identify high-potential drug candidates and eliminate as early as possible those with inferior potency, poor pharmacokinetic properties, and toxicity problems, so that these deficient drug candidates do not incur the high costs of clinical trials. During the last 5 years, a decision-making go/no-go strategy has been introduced into the drug discovery process, using pharmacokinetic principles to minimize the risks and maximize the benefits of selecting superior drug candidates.Pharmacological deficiencies are related in part to pharmacokinetic properties. To understand this process, a brief review of pharmacokinetic properties including oral bioavailability, half-life, absorption, clearance, and volume of distribution is presented. We examine in vitro — in vivo (human) and/or in vivo (animals) — in vivo (human) correlations for several of these pharmacokinetic properties, followed by a discussion of how this preclinical information is collected and used in drug discovery at the various stages to select drug candidates. Finally, we summarize how these methods are used to make go/no-go decisions in each step of the drug discovery process.

Antagonism by ETYA of the effects of leukotrienes on ileum and lung parenchymal strips independent of effects on arachidonic acid metabolism

5,8,11,14-Eicosatetraynoic acid (ETYA), a compound which inhibits both the cyclooxygenase and lipoxygenase pathways of arachidonic acid metabolism, antagonized the contraction of segments of guinea-pig ileal longitudinal muscle produced by SRS-A (IC50 = 2.73 microM). This activity was unaffected by pretreatment of the tissues with 10 microM indomethacin. Phenidone, another mixed cyclooxygenase-lipoxygenase inhibitor, was inactive. FPL-55712, an SRS-A antagonist, was a very potent inhibitor (IC50 = 0.011 microM). BW755C and NDGA nonselectively inhibited the contractions of the guinea-pig ileal longitudinal muscle induced by SRS-A or histamine. ETYA antagonized the contraction of the guinea-pig ileal strip produced by 6 nM synthetic LTC4 (IC50 = 9.3 microM). FPL-55712 demonstrated an IC50 of 0.3 microM in a similar series of experiments. ETYA, 1, 3 or 10 microM did not inhibit the contractions elicited by 0.5 microM of histamine. This was not a tissue-selective effect since 100 microM ETYA antagonized the LTC4-induced contraction of the guinea-pig lung parenchymal strip preparation. These data demonstrate that ETYA antagonized the contractile effect of the leukotrienes on tissues from the gastrointestinal tract and lung. Furthermore, the inability of indomethacin or phenidone to inhibit the contractile response suggests that antagonism by ETYA may occur by a mechanism independent of cyclooxygenase and lipoxygenase enzymes.