Method for predicting human intestinal first-pass metabolism of UGT substrate compounds

1. As intestinal glucuronidation has been suggested to generate the low oral bioavailability (F) of drugs, estimating its effects would be valuable for selecting drug candidates. Here, we investigated the absorption and intestinal availability (F_aF_g) in animals, and intrinsic clearance via UDP-glucuronosyltransferase (UGT) in intestinal microsomes (CL_int,UGT) for three drug candidates possessing a carboxylic acid group, in an attempt to estimate the impact of intestinal glucuronidation on F and select potential drug candidates with high F in humans.

2. The F_aF_g values of the three test compounds were low in rats and monkeys (0.16–0.51), and high in dogs (≥0.81). Correspondingly, the CL_int,UGT values were high in rats and monkeys (101–731 µL/min/mg), and low in dogs (≤?59.6 µL/min/mg). A good inverse correlation was observed between F_aF_g and CL_int,UGT, suggesting that intestinal glucuronidation was a major factor influencing F_aF_g of these compounds.

3. By applying this correlation to F_aF_g in humans using human CL_int,UGT values (26.9–114 µL/min/mg), compounds 1–3 were predicted to have relatively high F_aF_g.

4. Our approach is expected to be useful for estimating the impact of intestinal glucuronidation on F in animals and semiquantitatively predicting human F for drug candidates.

     

Differences in nonclinical pharmacokinetics between species and prediction of human pharmacokinetics of TAK‐272 (SCO‐272), a novel orally active renin inhibitor

In the search for orally available drugs, the prediction of human pharmacokinetics (PK) is essential for successfully selecting compounds that will be clinically useful. This report describes the selection of TAK‐272 (SCO‐272), a novel orally active renin inhibitor, as a clinical candidate via the detailed investigation of nonclinical PK data and human PK prediction. The bioavailability (BA) of TAK‐272 after oral administration to rats and monkeys was low, especially in fasted monkeys, and the systemic exposure of TAK‐272 was highly variable in monkeys. The results of mass balance studies in animals suggested that the absorbed TAK‐272 was largely eliminated by metabolism. In vitro studies revealed that TAK‐272 was mainly metabolized by CYP3A4/5 in humans, and it was a P‐glycoprotein substrate. PK analysis suggested that the factors responsible for the low BA were different in rats and monkeys. First‐pass hepatic extraction was high in rats, while the fraction absorbed from the gastrointestinal tract (F_a * F_g) was low in monkeys. It was predicted that humans would have a higher BA and a longer half‐life in the plasma compared with the animals by a simple calculation using intrinsic hepatic clearance in monkeys, which correlates well with human values for CYP3A4 substrates, and F_a * F_g in rats, which correlates relatively well with human values. TAK‐272 was finally selected as a clinical candidate based on the result of human PK prediction. The actual human PK after oral administration of TAK‐272 was comparable to the predicted profile and was preferable for clinical usage.     

Apparent Lack of Effect of P-Glycoprotein on the Gastrointestinal Absorption of a Substrate,Tacrolimus, in Normal Mice

Purpose. To study the contribution of P-glycoprotein (P-gp) to the oralabsorption of a substrate, tacrolimus, by comparing the extent and rateof bioavailability in normal and mdr1a knockout mice. Methods. Intravenous and oral (2 mg/kg) blood concentration data oftacrolimus in normal and knockout mice were obtained from a studyby K. Yokogawa et al. in Pharm. Res. 16:1213-1218 (1999). Meanbioavailability (F), mean hepatic first-pass extraction ratio (F_h), meanbioavailability rates, mean oral clearance, and mean total hepaticintrinsic clearance were calculated using standard pharmacokinetic methods. Results. The mean F of tacrolimus (an apparently highly permeablecompound) was increased from 0.22 in normal mice to 0.72 in knockoutmice. These values were consistent with mean predicted E_h (based onintravenous data) of 0.77 and 0.27 in normal and knockout mice,respectively. Great similarity in the relative bioavailability profile (suchas short T_max) between normal and knockout mice was also found. Meanoral clearance and mean total or unbound hepatic intrinsic clearance oftacrolimus in knockout mice were found to be about 10 times lowercompared to those in normal mice. Conclusions. The above results suggest an apparent lack of effect ofP-gp on the gastrointestinal absorption of tacrolimus in normal miceunder the study condition. It is postulated that the effect of P-gp onthe rate and extent of oral absorption should be more pronounced forthose more slowly or incompletely absorbed drugs (i.e., drugs withrelatively low permeabilities) as illustrated by talinolol in humans. Theclearance data also suggest a very dominant role of P-glycoprotein incontrolling the rate of hepatic metabolism of tacrolimus in normalmice, and P-glycoprotein may serve as an effective efflux pump fordirect transport of metabolites formed in hepatocytes into the bloodcirculation.     

Bioequivalence of a Highly Variable Drug: An Experience with Nadolol

Purpose. To assess the bioequivalence of nadolol 40mg and 160mg tablets (Zenith-Goldline Pharmaceuticals) using Corgard^® 40mg and 160mg tablets (Bristol-Meyers Squibb) as reference products, to estimate the effect of food in the gastrointestinal tract on nadolol bioavailability, and to evaluate the effectiveness of standard pharmacokinetic metrics AUC_t, AUC, and C_max in bioequivalence determinations. Methods. Four bioequivalence studies were conducted as described in the FDA Guidance. Four additional studies of varying designs were conducted to establish bioequivalence of the 40mg tablet in terms of C_max. Results. Fasted and food-effect studies of the 160mg tablet clearly established bioequivalence and revealed an unexpected reduction in nadolol bioavailability from test and reference products in the presence of food. The food-effect study of the 40mg tablet (80mg dose) revealed a similar reduction in bioavailability from each product. Fasted studies of the 40mg tablet (80mg dose) established bioequivalence in terms of AUC_t and AUC. However, C_max criteria proved extremely difficult to meet in the initial 40mg fasted study because of the large variability, leading to additional studies and ultimately requiring an unreasonable number of subjects. Conclusions. Final results clearly established bioequivalence of both strengths and characterized an unexpected food effect which did not appear to be formulation-related. However, the C_max of nadolol is only slightly sensitive to absorption rate and the relatively large variability of C_max reduces its effectiveness as a bioequivalence metric. Findings suggest that bioequivalence criteria for highly variable drugs should be reconsidered.     

Theorems and implications of a model-independent elimination/distribution function decomposition of linear and some nonlinear drug dispositions. III. Peripheral bioavailahility and distribution time concepts applied to the evaluation of distribution kinetics

Disposition decomposition analysis (DDA) is applied to evaluate the rate and extent of drug delivery from the sampling compartment to the peripheral system, i.e., peripheral bioavailability. Four parameters are introduced which are useful in quantifying peripheral bioavailability. The compounded peripheral bioavailability, F_comp,is the ratio between the total compounded amount of drug transferred to the peripheral system and the injected dose, D.The AUCperipheral bioavailability, F_AUC,is the ratio between the area under the amount vs.time curves for the peripheral system and the sampling compartment. The distribution time t_d,is the time following an i.v. bolus at which the net transfer of drug to the peripheral system reverses in direction. The maximum peripheral bioavailability, F_max,is the maximum fraction of an i.v. bolus dose that is present in the peripheral system at any one time. Equations are derived which permit estimation of those parameters from drug concentrations in the sampling compartment. Simple algorithms and a computer program are provided for estimating F_comp, F_AUC, t_d, F_max,and other parameters relevant to DDA for drugs that exhibit a linear polyexponential bolus response. Estimates of E_comp, F_AUC, t_d,are presented for several drugs.     

Effects of fenugreek,garden cress,and black seed on theophylline pharmacokinetics in beagle dogs

Abstract

Context: Herb–drug interactions are a serious problem especially for drugs with a narrow therapeutic index, taking into consideration that herbal medicines are commonly used in various parts of the world.

Objective: The present study investigates the effect of fenugreek, garden cress, and black seed on the pharmacokinetics of theophylline in beagle dogs.

Materials and methods: Beagle dogs received theophylline (200?mg) orally and blood samples were withdrawn at different time intervals (0.33, 0.66, 1.0, 1.5, 2, 3, 4, 6, 8, 12, 24, and 30?h). After a suitable washout period, each herb was given orally at doses of 25, 7.5, and 2.5?g, twice daily for 7 d. On the eighth day, theophylline was re-administrated orally and blood samples were collected. Plasma concentrations of theophylline were determined using HPLC and pharmacokinetic parameters were calculated using a non-compartmental analysis.

Results: Treatment with fenugreek (25?g, orally) lead to a decrease in C_max and AUC_0–t of theophylline of about 28% (p?<?0.05) and 22% (p?<?0.05), respectively, with no significant changes in T_1/2λ compared with the baseline values. Garden cress caused a decrease in C_max to a lesser extent and delayed T_max of theophylline (2.10?±?0.24?h versus 3.40?±?0.74?h), while AUC_0–∞ increased by 37.44%. No significant effect was observed for the black seed treatment on theophylline disposition as measured by C_max, T_max, AUC_0–∞, and CL/F.

Discussion and conclusion: The concurrent use of fenugreek or garden cress alters theophylline pharmacokinetic behavior in an animal model. This could represent a modulation in cytochrome P450 activity, which is responsible for theophylline metabolism in beagle dogs. Further confirmation of these results in humans will warrant changes in theophylline dosing before the co-administration of such herbs.     

In Vitro-In Vivo Evaluation of a Controlled Release Buccal Bioadhesive Device for Oral Drug Delivery

Purpose. To investigate the use of buccal bioadhesive device in targeting controlled drug delivery to the gastrointestinal tract. Methods. A three-leg crossover study was designed to evaluate the application of buccal bioadhesive device for providing controlled drug delivery to the gastrointestinal tract of a model drug cyanocobalamin in four healthy adult male beagle dogs. Results. In vitro dissolution studies using deionized water as the medium indicated that 100% of the drug was released within 15 min from a immediate release oral capsule formulation, whereas 90% of the drug was released within a period of 18 hrs from a buccal bioadhesive device formulation. Drug release from the buccal bioadhesive devices appeared to follow Higuchi's square root of time dependent model. The terminal half-life of the drug following I.V. administration in four dogs was found to be 16.4 ± 2.4 hrs. Following immediate release oral capsule administration of the drug C_max, t_max and bioavailability were 2333 ± 1469 ng/L, 2.5± 1.0 hrs and 14.1 ± 7.9%, respectively. Following buccal bioadhesive device administration of the drug C_max, t_max and bioavailability were 4154 ± 1096 ng/L, 11 ± 1.2 hrs and 35.8 ± 4.1%, respectively. Significantly higher bioavailability of the drug was observed with the buccal bioadhesive device administration when compared to the immediate release oral capsule. Conclusions. The buccal bioadhesive device appears to improve the oral bioavailability of cyanocobalamin by providing controlled delivery of the drug to the gastrointestinal tract.     

Evaluation of Capacity-Limited First-Pass Effect through Liver by Three-Points Sampling in Portal and Hepatic Veins and Systemic Artery

Purpose. The three-points method was newly developed by sampling the portal and hepatic veins and systemic artery. A model of hepatic local disposition with the Michaelis-Menten elimination was proposed to explain the concentration dependency of the hepatic recovery ratio (F _H). Methods. 5-fluorouracil (5-FU) was selected as a model drug. 5-FU was administered orally 90 min after its intraarterial dose. Blood specimens in both femoral artery and hepatic vein were sampled after intraarterial dose, and blood specimens in both femoral artery and portal vein were taken after oral administration. Results. It was shown that F _H increased with an increase in the input drug concentration into the liver. The mean absorption time (MAT) estimated by nonlinear analysis agreed with the mean local absorption time (t _a) whereas MAT by linear analysis was significantly smaller than t _a. Conclusions. The three-points method was newly developed, and the proposed nonlinear model explained well the capacity-limited elimination of 5-FU through the liver. MAT by the nonlinear analysis was in good agreement with t _a.     

Raloxifene glucuronidation in liver and intestinal microsomes of humans and monkeys: contribution of UGT1A1, UGT1A8 and UGT1A9

1.?Raloxifene is an antiestrogen that has been marketed for the treatment of osteoporosis, and is metabolized into 6- and 4′-glucuronides by UDP-glucuronosyltransferase (UGT) enzymes. In this study, the in vitro glucuronidation of raloxifene in humans and monkeys was examined using liver and intestinal microsomes and recombinant UGT enzymes (UGT1A1, UGT1A8 and UGT1A9).

2.?Although the K_m and CL_int values for the 6-glucuronidation of liver and intestinal microsomes were similar between humans and monkeys, and species differences in V_max values (liver microsomes, humans?>?monkeys; intestinal microsomes, humans?<?monkeys) were observed, no significant differences were noted in the K_m or S₅₀, V_max and CL_int or CL_max values for the 4′-glucuronidation of liver and intestinal microsomes between humans and monkeys.

3.?The activities of 6-glucuronidation in recombinant UGT enzymes were UGT1A1?>?UGT1A8?>UGT1A9 for humans, and UGT1A8?>?UGT1A1?>?UGT1A9 for monkeys. The activities of 4′-glucuronidation were UGT1A8?>?UGT1A1?>?UGT1A9 in humans and monkeys.

4.?These results demonstrated that the profiles for the hepatic and intestinal glucuronidation of raloxifene by microsomes were moderately different between humans and monkeys.     

Nilotinib preclinical pharmacokinetics and practical application toward clinical projections of oral absorption and systemic availability

Nilotinib is a highly potent and selective bcr‐abl tyrosine kinase inhibitor used for the treatment of patients who are in the chronic and accelerated phases of Philadelphia chromosome‐positive (Ph+) chronic myeloid leukemia (CML). Nilotinib preclinical data and its use for practical predictions of systemic exposure profiles and oral absorption are described. The systemic clearance (CL) of nilotinib was relatively low in rodents with a value of less than 25% of hepatic blood flow (Q_H), while it was moderate in monkeys and dogs (CL /Q_H = 32–35%). The steady state volume of distribution (V_ss) ranged from 0.55 to 3.9 l/kg across the species tested. The maximum concentration (C_max) of nilotinib occurred at 0.5–4 h and the bioavailability was moderate (17–44%). The plasma protein binding was high (> 97.5%) in preclinical species and humans. The human CL (~ 0.1 l/h/kg) and V_ss (~2.0 l/kg) were best predicted by the rat–dog–human proportionality method and allometric scaling method, respectively. The human intravenous pharmacokinetic profile was projected by the Wajima ‘C_ss‐MRT’ method. The predicted micro‐constants from human intravenous profiles were incorporated into the advanced compartmental absorption and transit model within the GastroPlus program to simulate the oral concentration–time curves in humans. Overall, the simulated oral human pharmacokinetic profiles showed good agreement with observed clinical data, and the model predicted that the C_max, AUC, t_1/2, V_z/F and CL/F values were within 1.3‐fold of the observed values. The absolute oral bioavailability of nilotinib in healthy humans was predicted to be low (< 25%). Copyright © 2012 John Wiley & Sons, Ltd.     

A Review of: “Statistics: Concepts and Controversies,Sixth Edition,by D. S. Moore and W. I. Notz (Eds.)”

Contrasts were evaluated for the maximum blood or plasma concentration (C _max) of drugs measured after repeated and single oral administrations. Variances C _max of were calculated and also simulated for a single drug as well as the comparison of two formulations, i.e., for the analysis of investigations of both bioavailability and bioequivalence. The coefficient of variation (C V) of C _max was higher in the steady state than after a single drug administration when the variability of the disposition rate constant (k) was substantially larger than that of the absorption rate constant (k_a )In turn, the CV of C _max was substantially lower following repeated than after single drug administration when the variability of k_a dominated that of k.The latter condition often prevails in practice since the relative variation of absorption rates generally substantially exceeds that of clearance (the latter being proportional to k) The statistical insensitivity is superimposed on the low kinetic sensitivity exhibited by C _maxfollowing repeated drug administrations. Consequently, bioequivalence trials conducted in the steady state generally permit a declaration of equivalence even between drug products that have very different absorption rates     

Intra- and Intersubject Variability: Mixed-Effects Statistical Analysis of Repeated Doses of an Angiotensin Converting Enzyme Inhibitor,CGS 16617

The intrasubject and intersubject variabilities for CGS 16617, an angiotensin converting enzyme inhibitor, were evaluated in an open-label, repeat single-dose bioavailability trial. Eight healthy male volunteers each received a 20-mg oral dose of CGS 16617 as an aqueous solution on four separate occasions. Components of variance were evaluated for a mixed-effects statistical model in which subjects were regarded as a random factor. While intersubject variability was statistically significant (P < 0.05) for all pharmacokinetic variables measured, AUC, C _max, t _1/2, and t _max, its contribution to the total observed variability was relatively small for AUC, t _1/2 and t _max. The proportion of variation due to intrasubject variability was 70, 19, 61, and 72% for AUC, C _max, t _1/2, and t _max, respectively. Ramifications of the large intrasubject source component of variability as related to bioavailability trials and biological variation are discussed.     

蒙花苷磷脂复合物的制备及其药动学评价

目的 制备蒙花苷磷脂复合物，研究其药动学性质。方法 溶剂挥发法制备蒙花苷磷脂复合物，以复合率为评价指标，通过正交试验优化制备工艺。SD大鼠灌胃给予蒙花苷磷脂复合物，HPLC测定蒙花苷的血药浓度。结果 采用优化后的磷脂复合物制备工艺，复合率接近100%。X射线衍射结果表明蒙花苷以无定型状态存在于磷脂复合物中，在水和正辛醇中的表观溶解度显著增大。药动学结果显示，蒙花苷磷脂复合物的t_max，C_max，AUC_0-t等参数与蒙花苷原料药相比具有显著性差异，口服吸收生物利用度提高1.11倍。结论 磷脂复合物改善了蒙花苷的溶解性质，口服吸收生物利用度得到明显提高。     

A simplified approach to predict CYP3A-mediated drug–drug interactions at early drug discovery: validation with clinical data

Abstract

1. The present study evaluates which factors should be incorporated into a simplified approach to reasonably predict CYP3A-mediated drug–drug interaction (DDI) at an early drug discovery stage.

2. CYP3A IC₅₀ values were obtained using human liver microsomes (HLM) and hepatocytes. Plasma and microsomal protein binding and in vitro hepatocyte partition coefficient (K_p) were also determined for 10 drugs. Therapeutic human maximum plasma concentrations (C_max) were retrieved from the literature. DDI predictions were performed using an equation incorporating the fraction of the substrate metabolized by CYP3A with the total or free plasma C_max, with or without correction for hepatocyte K_p.

3. Based on the K_i data from HLM, the use of total C_max provided a prediction of DDI within 2-fold of the observed clinical values for 9 out of 10 drugs.

4. In comparison, free drug corrections for both C_max and K_i values from HLM led to an underprediction of DDI (>3-fold error for five drugs).

5. Data from hepatocytes showed, in general, lower prediction accuracy than data from HLM.

6. CYP3A-mediated DDIs can be predicted with a high level of accuracy based on K_i estimates from HLM data and the total therapeutic plasma C_max of the inhibitors. This approach should be widely applicable to the assessment of clinically significant DDIs risk in early drug discovery programs     

Comparison of Zafirlukast (Accolate®) Absorption After Oral and Colonic Administration in Humans

Purpose. This study characterized the gastrointestinal (GI) absorptionof zafirlukast after oral and colonic administration in humans. Methods. Five healthy subjects received zafirlukast solution (40 mg)orally and via an oroenteric tube into the colon in a randomized,crossover fashion. Two additional subjects were dosed into the distalileum. Serial blood samples were obtained and plasma concentrationswere quantitated by HPLC. Results. Mean ± SD pharmacokinetic parameters after oral vs. colonicadministration were: AUC of 2076 ± 548 vs. 602 ± 373 ng*h/mL,respectively, and C_max of 697 ± 314 vs. 194 ± 316 ng/mL, respectively.Mean colon:oral AUC and C_max were 0.29 and 0.30, respectively.Median t_max values were 2.0 and 1.35 hr after oral and colonicadministration. First-order absorption rate constants (K_a and K_ac) wereestimated from a two-compartment model with first-order elimination.K_ac:K_a was <0.5 in 4 of the 5 subjects dosed in the colon. Conclusions. Zafirlukast was absorbed at multiple sites in the GI tract.The rate and extent of zafirlukast absorption was less after colonicthan oral administration. Zafirlukast was significantly absorbed in thedistal ileum. This study demonstrated that gamma scintigraphy, digitalradiography, and fluoroscopy can be used to track the movement andconfirm the location of the oroenteric tube in the GI tract.     

Evaluation of Using Dog as an Animal Model to Study the Fraction of Oral Dose Absorbed of 43 Drugs in Humans

Purpose. To conduct a retrospective evaluation of using dog as ananimal model to study the fraction of oral dose absorbed (F) of 43drugs in humans and to briefly discuss potential factors that mighthave contributed to the observed differences in absorption. Methods. Mean human and dog absorption data obtained under fastedstate of 43 drugs with markedly different physicochemical andpharmacological properties and with mean F values ranging from 0.015 to1.0 were obtained from the literature. Correlation of F values betweenhumans and dogs was studied. Based on the same references, additionalF data for humans and rats were also obtained for 18 drugs. Results. Among the 43 drugs studied, 22 drugs were virtuallycompletely absorbed in both dogs and humans. However, the overallcorrelation was relatively poor (r² = 0.5123) as compared to the earlier ratvs. human study on 64 drugs (r² = 0.975). Several drugs showed muchbetter absorption in dogs than in humans. Marked differences in thenonliner absorption profiles between the two species were found forsome drugs. Also, some drugs had much longer T_max values andprolonged absorption in humans than in dogs that might be theoreticallypredicted. Data on 18 drugs further support great similarity in F betweenhumans and rats reported earlier from our laboratory. Conclusions. Although dog has been commonly employed as ananimal model for studying oral absorption in drug discovery anddevelopment, the present study suggests that one may need to exercise cautionin the interpretation of data obtained. Exact reasons for the observedinterspecies differences in oral absorption remain to be explored.     

Correlation of Plasma Clearance of 54 Extensively Metabolized Drugs Between Humans and Rats: Mean Allometric Coefficient of 0.66

Purpose. To evaluate the distribution of allometric exponents for relationship of total plasma clearance of 54 extensively metabolized drugs, with wide-ranging linear clearance values, between humans and rats, to provide a rationale for the observed data, and to discuss potential significance of the findings. Methods. Human and rat plasma clearance values of 54 drugs with markedly different physicochemical properties were obtained from the literature. Standard allometric analysis was performed for each drug using both rat and human data. Unbound vs. total plasma clearances were obtained for 15 out of 54 drugs and their correlations between humans and rats were compared. Results. The mean ± SD of the allometric exponent for the 54 drugs studied is 0.660 ± 0.190. The median clearance ratio based on unit body weight is 7.41 and the median exponent is 0.645. Excluding two outliers the correlation coefficient of plasma clearance between humans and rats was 0.745 (p < 0.0001). For the 15 drugs, use of unbound plasma clearance approach seems to significantly improve the correlation coefficient compared to total plasma clearance (0.940 vs. 0.841). Conclusions. The present study indicates that on average, humans and rats may eliminate extensively metabolized drugs at a rate similar to that expected from the allometric or body surface area relationship of basal metabolic rate between the two species. A simple statistical distribution hypothesis is used to rationalize the species difference in plasma drug clearance. Rat may serve as an useful animal model to predict (unbound) plasma clearance of drugs in humans.     

双膦酸盐类药物的药动学及药物相互作用研究进展

双膦酸盐类药物用于预防与治疗骨吸收相关疾病,包括第1代的依替膦酸二钠、替鲁膦酸二钠、氯膦酸二钠,第2代的帕米膦酸二钠、阿仑膦酸钠,第3代的利塞膦酸钠、伊班膦酸钠、唑来膦酸等。该类药物口服达峰时间较短,吸收率偏低,生物利用度很低;骨结合部分的半衰期较长,药物留存时间也较长。与双膦酸盐联用产生相互作用的药物主要为：含有二价阳离子药物、其他双膦酸盐药物、华法林、苯妥英、非甾体类抗炎药、抗癌药、激素、H2受体拮抗剂以及肾毒性药物。从药动学角度解释双膦酸的药物相互作用,以避免不良反应发生。     

Differences in platelet serotonin transporter sites between African-American tobacco smokers and non-smokers

Rationale. The serotonin transporter (5HTT) regulates the magnitude and duration of serotonergic neurotransmission. Although nicotine and other constituents of tobacco smoke may influence serotonin turnover among animals, few studies have examined whether smoking is associated with alteration in 5HTT in humans. Objective. We investigated whether tobacco smokers and non-smokers differed in platelet tritiated paroxetine binding, a measure of 5HTT sites, and whether severity of nicotine dependence (ND) was related to 5HTT measures. Methods. Tritiated paroxetine binding sites on platelets were assayed in 26 African-American smokers and 30 non-smokers. Severity of smoking was assessed using the Fagerstrom Test for Nicotine Dependence (FTND). Relationships between FTND scores and maximum number of transporter sites (B_max) and affinity constant (K_d) of paroxetine binding were determined. Results. B_max values showed a significant negative correlation with FTND scores (rho=−0.28, P<0.01). Notably, smokers with higher ND had significantly lower B_max compared to those with lower ND and non-smokers; the latter two groups did not differ in B_max (F=3.92, P<0.05). Smokers scored higher on impulsivity than non-smokers, however, behavioral variables did not influence the relationship of smoking with B_max. Age, gender and K_d values were not associated with smoking or B_max. Conclusions. Smoking, in particular higher nicotine dependence, appears to be correlated with decreased density of platelet 5HTT sites in African-Americans. The nature of the relationship and whether similar changes occur in the brain merit further investigation. Electronic Publication     

High Variability in Drug Pharmacokinetics Complicates Determination of Bioequivalence: Experience with Verapamil

Purpose. For the assessment of bioequivalence it is assumed that drug clearance in each subject on each of the study days is the same and any observed differences in AUC and/or Cmax between a brand and generic formulation are due to differences in bioavailability. We hypothesized that this assumption was invalid for highly variable drugs such as verapamil and tested it by comparing bioavailability for the brand vs itself. Methods. To avoid any contribution from potential formulation differences, we evaluated bioavailability for Isoptin SR 240 mg tablets in 9 healthy volunteers on 2 occasions separated by 1 week as part of a larger study. A validated HPLC assay was used to measure serial blood samples over 36 hours. Results. The AUC_0–t varied 3.8 fold among subjects and 5/9 subjects had >30% difference in AUC_0–t on the 2 days. After log transformation, the mean AUC_0–t ± %cv (ng·h/mL) on Occasion 1 (878 ± 38) was 23% greater (p = 0.031) than on Occasion 2 (713 ± 41). The 90% confidence interval of Occasion 1/Occasion 2 was 106–143%. The Cmax varied >9 fold (30–278 ng/mL) among subjects. The intra-subject difference between days ranged from –46% to +298%. The 90% confidence interval was 72–152% for Cmax. Since the same lot of Isoptin was used in the same subjects on 2 occasions, the observed differences must be due to biological variability in verapamil pharmacokinetics, not formulation differences. Conclusions. The intra-subject biological variability complicates bio-equivalence assessment and can lead to an erroneous assumption of bioinequi valence.