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.

     

Method to Estimate the Rate and Extent of Intestinal Absorption in Conscious Rats Using an Absorption Probe and Portal Blood Sampling

Purpose. A variety of methods exist which determine the rate and extent of intestinal absorption. The method described here employs an internal absorption reference probe and portal blood sampling in unanesthetized rat. Methods. Theophylline and tritiated water were selected as absorption reference probes since they are quantitatively absorbed in conscious rat. The fraction of an intestinal dose which reaches portal blood was determined from the resulting portal-systemic blood concentration gradients of the drug relative to the absorption probe. The absorption probes provide a means to calculate the drug mass reaching portal blood without the need of measuring the portal blood flow rate. The technique was evaluated with verapamil and a well-absorbed 5-lipoxygenase inhibitor, A-79035. Results. The fraction of an intrajejunal dose of A-79035 reaching the portal vein (F_G) was 0.86 using theophylline as the absorption probe. Verapamil, which is susceptible to extensive hepatic first-pass elimination, was completely absorbed (F_G = 0.98) within 1 hour, but was only 21.4% bioavailable. Absorption rate constants, estimated from initial appearance rates in portal blood, were used to monitor factors that affect drug absorption. For example, with a dose solution containing 30% PEG-400, the absorption rate constants of theophylline and A-79035 were significantly reduced. Anesthesia reduced the absorption rate constant for theophylline in rats by 40% compared to conscious animals. Conclusions. The technique detailed here allows reliable, direct measurement of intestinal absorption which may assist in characterizing oral dosing for novel therapeutic agents.     

Pharmacokinetics of single oral and multiple intravenous and oral administration of acebutolol enantiomers in a rat model

Acebutolol (AC), is a chiral, β -adrenergic blocking agent which possesses partial agonist activity and is metabolized to an equipotent chiral metabolite, diacetolol (DC). The enantiomeric disposition of AC is reported following racemic administration as a single oral (p.o., 50 mg kg⁻¹) or as a multiple thrice daily intravenous (i.v.) or p.o. dosing for four days in male Sprague–Dawley rats (n =6). Enantiomeric concentrations of AC and DC in plasma and urine were determined using a stereospecific HPLC assay. The bioavailabilities of R- and S-enantiomer were 0.40 and 0.39 after single dose administration of AC respectively. These values were increased to 0.51 and 0.53 after multiple dosing. Although no significant differences were found in AUC_0–∞ after single i.v. as compared with AUC_0–τ after multiple i.v. dosing of AC, the 39 and 45% increase in mean AUC_0–τ were found after multiple p.o. dosing over the corresponding AUC_0–∞, for the single p.o. dose of AC for R- and S-enantiomer, respectively. The disposition of DC as well as the urinary excretion of metabolite was stereoselective in favor of R-enantiomer after oral administration of AC. These results indicate that AC enantiomers have low availability and moderate extraction through the first-pass metabolism in a rat model. The higher AUC values after p.o. multiple dosing may suggest a saturable first-pass metabolism of AC. © 1998 John Wiley & Sons, Ltd.     

In vitro–in vivo extrapolations to evaluate the effect of concomitant drugs on tacrolimus (FK506) exposure

The effect of concomitant drugs having a cytochrome P450 (CYP) 3A inhibitory potency on tacrolimus exposure was predicted from in vitro metabolism results. In this study, the IC₅₀ values of concomitant drugs on the formation of M‐I, the major metabolite of tacrolimus, were determined, and the effect on oral exposure (AUC_p.o.) of tacrolimus was assessed from static models. When an absorbed fraction (F_a) of 0.97, intestinal wall availability (F_g) of 0.27 and fraction metabolized by CYP3A (f_m(CYP3A)) of 0.8 were used, the least bias was observed for the prediction of the AUC_p.o. of tacrolimus. The relationship of the IC₅₀ values of 11 inhibitors between tacrolimus and typical CYP3A substrates (midazolam and testosterone) was also analysed. A strong correlation was found between the IC₅₀ values of tacrolimus and typical CYP3A substrates (r² ≥ 0.85). The predictability of the effect of inhibitors on tacrolimus AUC_p.o. was investigated based on the same static models with the use of published IC₅₀ values for midazolam and testosterone. The bias for the prediction of tacrolimus AUC_p.o. was minimal with the use of IC₅₀ values determined using tacrolimus itself as a substrate. These results suggest that tacrolimus itself is still the best choice for predicting the AUC_p.o. of tacrolimus, although our findings suggest that midazolam or testosterone may be used instead of tacrolimus to estimate roughly (predicted AUC_p.o. within an approximately 2‐fold range of observed values) the effect of CYP3A inhibitors on the tacrolimus AUC_p.o.. Copyright © 2015 John Wiley & Sons, Ltd.     

Pharmacokinetics of dietary cancer chemopreventive compound dibenzoylmethane in rats and the impact of nanoemulsion and genetic knockout of Nrf2 on its disposition

The pharmacokinetic disposition of a dietary cancer chemopreventive compound dibenzoylmethane (DBM) was studied in male Sprague‐Dawley rats after intravenous (i.v.) and oral (p.o.) administrations. Following a single i.v. bolus dose, the mean plasma clearance (CL) of DBM was low compared with the hepatic blood flow. DBM displayed a high volume of distribution (Vss). The elimination terminal t_1/2 was long. The mean CL, Vss and AUC_0?∞/dose were similar between the i.v. 10 and 10 mg/kg doses. After single oral doses (10, 50 and 250 mg/kg), the absolute oral bioavailability (F^*) of DBM was 7.4%–13.6%. The increase in AUC was not proportional to the oral doses, suggesting non‐linearity. In silico prediction of oral absorption also demonstrated low DBM absorption in vivo. An oil‐in‐water nanoemulsion containing DBM was formulated to potentially overcome the low F^* due to poor water solubility of DBM, with enhanced oral absorption. Finally, to examine the role of Nrf2 on the pharmacokinetics of DBM, since DBM activates the Nrf2‐dependent detoxification pathways, Nrf2 wild‐type (+/+) mice and Nrf2 knockout (?/?) mice were utilized. There was an increased systemic plasma exposure of DBM in Nrf2 (?/?) mice, suggesting that the Nrf2 genotype could also play a role in the pharmacokinetic disposition of DBM. Taken together, the results show that DBM has low oral bioavailability which could be due in part to poor water solubility and this could be overcome by a nanotechnology‐based drug delivery system and furthermore the Nrf2 genotype could also play a role in the pharmacokinetics of DBM. Copyright © 2010 John Wiley & Sons, Ltd.     

Analysis of the Complicated Nonlinear Pharmacokinetics of Orally Administered Telmisartan in Rats Using a Stable Isotope-IV Method

This study aimed to kinetically analyze the nonlinear absorption and systemic exposure of telmisartan (TEL) after oral administration to rats by using a stable isotope-IV method. Rats were orally administered different dose of TEL, followed by the intravenous injection of 0.005 mg/kg of deuterium-labeled TEL (TEL-d3). Assuming that TEL-d3 shows same pharmacokinetic properties with TEL, systemic clearance (CL_tot), oral bioavailability (F_oral), and intestinal and hepatic availability (F_a*F_g, F_h) of TEL were calculated in each individual rat. AUC_po of TEL increased disproportionately with dose and showed a sigmoid-type relation, indicating the involvement of multi-nonlinear processes in oral absorption of TEL. F_a*F_g of TEL increased with dose at the low-dose range while decreased at the high-dose range. In contrast, F_h increased and CL_tot decreased significantly in the middle range (2 to 6 mg/kg). As main factors of nonlinearity, saturations of solubility, efflux transport in the intestine, and the hepatic uptake of TEL were indicated. In conclusion, this study demonstrated a high possibility of a stable isotope-IV method to characterize complicated pharmacokinetic properties of oral drugs in animals, which can help to consider the future risks in their clinical use.     

鸡尾酒法评价石蒜对大鼠细胞色素P450酶活性的影响

目的 运用鸡尾酒法评价石蒜对细胞色素P450酶活性的影响。方法 将大鼠随机分为对照组和石蒜低、高剂量组。对照组给予生理盐水，石蒜低、高剂量组大鼠灌胃给药0.5，1.0 g·kg^-1石蒜，连续给药15 d。然后第16天给予探针药物，UPLC-MS/MS检测探针药浓度。结果 与对照组对比，石蒜低剂量组和高剂量组的安非他酮AUC_（0-t）、C_max都显著升高（P<0.05），CL_z/F显著降低（P<0.05）。与对照组对比，石蒜高剂量组的美托洛尔、咪达唑仑和非那西丁AUC_（0-t）显著降低（P<0.05）、CL_z/F显著升高（P<0.05），而低剂量组与对照组比较差异无统计学意义。与对照组对比，石蒜组的甲苯磺丁脲AUC_（0-t）、CL_z/F差异无统计学意义，C_max显著降低（P<0.05）。结论 石蒜能抑制大鼠CYP2B1酶活性，诱导大鼠CYP2D1、CYP3A2和CYP1A2酶活性，稍有诱导大鼠CYP2C11酶活性的作用。     

Renal Clearance of a Recombinant Granulocyte Colony-Stimulating Factor,Nartograstim, in Rats

Purpose. To clarify the role of the kidney in the elimination of a recombinant human granulocyte colony-stimulating factor, nartograstim, we have investigated its pharmacokinetics in rats with renal failure. Methods. The steady-state clearance (CL_ss) were determined by the intravenous infusion for 4 hr to unilateral renally-ligated and cisplatin-treated rats, whose renal functions were about 50 and 10 % of controls, respectively. Results. CL_ss of nartograstim (27 ml/hr/kg) in the renally-ligated rats at a high infusion rate was significantly lower (25%) than in control rats (p<0.05). CL_ss in these rats, at a low infusion rate was 95 ml/hr/kg, 14 % lower than in control rats. The saturable CL_ss in these rats, 68 ml/hr/kg, was not significantly different from control rats (75 ml/hr/kg, p>0.05). Also, CL_ss in cisplatin-treated rats with extensive renal failure, at a high infusion rate, decreased to 57 % of controls. Furthermore, the total body clearances (CL_tot) of nartograstim after bolus intravenous administration to renally-ligated and cisplatin-treated rats were reduced to 33–49 % of controls. Conclusions. These results suggest that the kidney may be responsible for 40– 50 % of the nonsaturable clearance of nartograstim. Thus, the kidney should make a major contribution to the elimination of nartograstim when rats are given a high dose of nartograstim, which saturates the receptor-mediated clearance.     

Kinetic Analysis of the Disposition of Insulin-Like Growth Factor 1 in Healthy Volunteers

Purpose. Insulin-like growth factor 1 (IGF-1) is predominantly bound to its specific binding proteins (IGFBPs) in circulating plasma. In the present study, pharmacokinetic analysis of IGF-1 was performed in healthy volunteers to characterize the effect of interactions with IGFBPs on IGF-1 disposition. Methods. Plasma concentration profiles of both free and bound IGF-1 were examined at several doses. An in vitro plasma protein binding was also analyzed. Results. The total body clearance (CL_total) for the free IGF-1 was much higher than the creatinine clearance, suggesting that the major elimination pathway is by a route other than renal glomerular filtration. The CL_total for the free IGF-1 exhibited a dose-dependent reduction whereas that for the sum of unbound and bound IGF-1 increased on increasing the dose. The data obtained fitted closely a one-compartment model that involved the binding and dissociation of IGF-1, as well as its biosynthesis and elimination. The estimated parameters suggest that IGF-1 exhibits high affinity binding to IGFBPs, the rate-limiting step in the overall elimination being the dissociation from IGFBPs. Conclusions. The saturation of both the plasma protein binding and elimination accounts for the nonlinear pharmacokinetic profile. The binding to IGFBPs markedly limits both the distribution and elimination of IGF-1.     

A theoretical examination of the effects of gut wall metabolism,hepatic elimination,and enterohepatic recycling on estimates of bioavailability and of hepatic blood flow

A model including two eliminating compartments (the liver and the gastrointestinal mucosa) and a noneliminating compartment (the blood or central compartment) was developed to predict the effects of hepatic elimination, gastrointestinal mucosal metabolism, and the occurrence of enterohepatic recycling of a drug and its metabolites on the area under the blood concentrationtime curve (AUC). Several limiting cases where complete absorption or complete or nonexistent enterohepatic recycling of a drug and its metabolite occurred were only considered. Under linear kinetic conditions, the occurrence of hepatic elimination and enterohepatic recycling of a drug and its metabolite in the absence of intestinal mucosal metabolism should affect only the area under the curve and not the availability for both the intraperitoneal and the oral dose. In the presence of intestinal mucosal metabolism, the area under the curve should change with different routes of administration; a larger area, hence a higher availability, should occur after intraperitoneal administration than after oral administration of the drug. For a drug which is eliminated solely by the liver, apparent hepatic flow can be estimated by the dose divided by the difference in the area under the curve for an intravenous dose and the area under the curve for the same intraperitoneal or oral dose. In the absence of gastrointestinal mucosal metabolism, the presence of enterohepatic recycling of a drug and its metabolite should not affect the estimation of apparent hepatic blood flow. However, when gastrointestinal mucosal metabolism is present, there should be an overestimation of hepatic flow when AUC_i.p. and AUC_i.v. are used and a slight underestimation of hepatic flow when AUC_i.v. and AUC_p.o. are used.     

Pharmacokinetics of mirodenafil and its two metabolites,SK3541 and SK3544, after intravenous and oral administration of mirodenafil to streptozotocin-induced diabetes mellitus rats

1. The area under the curve (AUC) of mirodenafil after intravenous administration in diabetes mellitus induced by streptozotocin (DMIS) rats was significantly smaller (by 28.0?%) than the control value, and the AUC_SK3541/AUC_mirodenafil ratio was significantly greater (by 130?%) in DMIS rats. This may be explained by the significantly faster hepatic CL_int of mirodenafil, owing to increased hepatic CYP1A, CYP2B1/2, CYP2D, and CYP3A expression, and a faster hepatic blood flow rate, compared with control values.

2. The AUC of mirodenafil after oral administration was comparable between DMIS and control rats, possibly because of the comparable intestinal CL_int, which may be attributable to increased CYP1A2 expression and decreased CYP2D expression in the intestines of DMIS rats.

     

Cysteine effects on the pharmacokinetics of etoposide in protein–calorie malnutrition rats: increased gastrointestinal absorption by cysteine

1. Protein–calorie malnutrition (PCM) occurs frequently in advanced cancer patients and has a profound impact on the toxicity of many drugs. Thus, the pharmacokinetics of etoposide were evaluated in control, control with cysteine (CC), PCM, and PCM with cysteine (PCMC) rats.

2. Etoposide was administered intravenously (2?mg/kg) or orally (10?mg/kg). Changes in hepatic and intestinal cytochrome P450s (CYPs) and effects of cysteine on intestinal P-glycoprotein (P-gp)-mediated efflux were also measured.

3. In PCM rats, the CL_NR (AUC_0–∞) of intravenous etoposide was significantly slower (greater) than that in controls, because of the significant decrease in the hepatic CYP3A subfamily and P-gp. In PCMC rats, the slowed CL_NR of etoposide in PCM rats was restored to the control level by cysteine treatment. PCMC rats showed a significantly greater AUC_{0–6 h} of oral etoposide than PCM rats, primarily because of the increased gastrointestinal absorption of etoposide as a result of the inhibition of intestinal P-gp by cysteine.

4. The gastrointestinal absorption of an oral anticancer drug, which is a substrate of P-gp, may be improved by co-administration of cysteine in advanced cancer patients if the present rat data can be extrapolated to patients.

     

Divergent effects of raloxifene HCI on the pharmacokinetics and pharmacodynamics of warfarin

Purpose. Evista® (raloxifene HCl) is a nonsteroidal selective estrogen receptor modulator that displays estrogen agonist effects on bone and lipid metabolism but estrogen antagonist effects on the breast and endometrium. The potential for drug-drug interaction between raloxifene and warfarin was assessed in 15 healthy postmenopausal women. Methods. Single doses of warfarin (20 mg) were administered prior to and during 2 weeks of dosing with raloxifene 120 mg/day. Each warfarin dose was followed by pharmacokinetic sampling and prothrombin time measurements. Results. Raloxifene administration resulted in 7.1% and 14.1% decreases in the clearance (CL_P/F) and 7.4% and 9.8% decreases in the volume of distribution (V_SS/F) of R- and S-warfarin, respectively (all p 0.05). In contrast to the slightly higher plasma concentrations of R- and S-warfarin, raloxifene reduced the maximum prothrombin time (PT_max) by 10% and the area under the PT versus time curve from 0-120 h (AUC_PT) by 8% (p < 0.01). Conclusions. Raloxifene administration may result in a small increase in systemic warfarin exposure that is associated with a diminution, not augmentation, of the pharmacodynamic effect. Due to the small magnitude of this effect, concomitant administration of raloxifene and warfarin is not likely to result in clinically significant drug-drug interaction.     

Pharmacokinetic quantification of the exchange of drugs between blood and cerebrospinal fluid in man

Summary Various parameters which may be useful in quantification of drug transit from blood into CSF and vice versa after a short duration infusion are compared here by recalculating previously published data from our group.Due to the slower entry into and elimination from the CSF compartment as compared to the central compartment, the ratio of drug concentrations in CSF and serum sampled at the same time increase with time after an infusion. Therefore, concentration quotients of simultaneously drawn blood and CSF are inadequate to characterise CSF penetration.The ratio of the areas under the concentration-time curves in a body fluid and serum (AUC_{body fluid}/AUC_S) is an established measure to quantify overall penetration from the central into a peripheral compartment. AUC_CSF/AUC_S is closely correlated with the quotient of the maximum CSF and serum concentrations (C_maxCSF/ C_maxS) (r_S=0.87, n=42, P<0.001) and with the rate constant of distribution in CSF (CL_in/V_CSF) (r_S=0.80, n=42, P<0.001).Since C_maxCSF/C_maxS depends on the mode of drug administration, it is suggested that AUC_CSF/AUC_S be used to quantify overall drug transit into CSF. CL_in/V_CSF is of use when CSF can only be sampled once, or when the velocity of the transit of a drug into CSF is to be described.The CSF exit rate constant (CL_out/V_CSF) characterises elimination from CSF independent of the elimination from serum and may be applied to estimate the formation rate of CSF; in the present study it averaged 20 ml/h.This work is dedicated to Prof. Dr. K. Felgenhauer, Head of the Department of Neurology, University of Göttingen, on the occasion of his 60^th birthday     

Up-Regulation of Carnitine Transporters Helps Maintain Tissue Carnitine Levels in Carnitine Deficiency Induced by Pivalic Acid

Purpose. Pivalic acid (PVA) forms conjugates with endogenous carnitine and enhances its excretion. The purpose of this study is to determine whether tissue carnitine levels decrease in parallel with plasma levels in carnitine deficiency induced by PVA. Methods. PVA was orally administered to rats for 5 days. Carnitine levels in plasma, liver, kidney, muscle, and heart were monitored. The tissue uptake clearance (CL_uptake) was determined in vivoby the integration plot method. Hepatocytes were prepared from control and PVA-treated rats, and the uptake of L-carnitine was determined. Results. Plasma concentrations of L-carnitine decreased as a result of the enhanced carnitine elimination as pivaloylcarnitine (PCN) when rats were treated with PVA. However, L-carnitine concentrations in liver, muscle, and heart remained relatively constant during the study period. CL_uptake increased in liver and muscle and, thus, the rate of carnitine uptake from plasma into these tissues did not change even at low plasma concentrations. This helps maintain carnitine levels in these tissues. Up-regulation of carnitine transporters is suggested to be a mechanism for the increased CL_uptake. Conclusions. In the carnitine deficiency state induced by PVA, increased CL_uptake owing to up-regulation of carnitine transporters is suggested to help maintain carnitine levels in some tissues.     

The Effect of In Vivo Dissolution,Gastric Emptying Rate,and Intestinal Transit Time on the Peak Concentration and Area-Under-the-Curve of Drugs with Different Gastrointestinal Permeabilities

Purpose. To theoretically investigate the impact of gastric emptying half-time, intestinal transit time and the time for 85% in vivo dissolution on the peak concentration and area-under-the curve of model drugs. Methods. Simulations were performed using mathematical models of gastrointestinal physiology and pharmacokinetics of model drugs with different gastrointestinal permeability. They were used to investigate the effect of different permutations of gastric emptying times, intestinal transit times, dissolution rates and effective permeabilities on the maximum plasma drug concentration and the area-under-the-curve of immediate release tablets relative to an oral solution (i.e., Cmax_tablet/ Cmax_solution and AUC_tablet/AUC_solution). Results. The higher the permeability of the drug, the more sensitive the Cmax ratio is to dissolution rate and gastric emptying rate. As the intestinal transit time becomes more rapid, the sensitivity to T85% dissolution time and gastric emptying half-time increases. There is less dependence for the AUC ratio on the gastric emptying time and dissolution rate. Conclusions. Under the assumptions of the models, the criterion of 85% dissolution in 15 minutes (T85%) for classifying a rapidly dissolving drug product is relatively conservative since the Cmax ratio exceeded 0.8 for a T85% dissolution time of one hour and a gastric emptying half-time faster than 0.2 hour over a wide range of permeabilities.     

Effects of Hyperlipidemia on the Pharmacokinetics of Nifedipine in the Rat

Purpose. The effect of hyperlipidemia on nifedipine pharmacokinetics was studied. The mechanisms by which hyperlipidemia affects pharmacokinetics of drugs are mainly undetermined. Hyperlipidemia may decrease the fraction of unbound drug in plasma and/or decrease intrinsic ability of the cytochrome P-450 systems due to excess membrane cholesterol. Hyperlipidemia is a primary risk factor for coronary artery disease leading to hypertension and ischemic heart disease, for which nifedipine, a calcium channel blocker, is used. Methods. Poloxamer 407 (P407)-induced hyperlipidemic rat model was used to study the effects of hyperlipidemia on the pharmacokinetics of nifedipine (6 mg kg^–1 given iv, ip and po). Total plasma cholesterol levels increased from 0.82–2.02 to 5.27–11.05 mmol L^–1 48 h post P407 administration (Ig kg^–1, ip). Protein binding studies were conducted by an ultrafiltration method. Results. Hyperlipidemia significantly decreased CL_TB by 38% and CL_TB/F by 45 and 42% following po and ip doses, respectively, thereby increasing AUC_0–, C_max and half-life. Absolute bioavailability and Vd_ss remained unchanged. AUC_0– was affected to the same extent in each route of administration, therefore, the effect was mainly systemic rather than presystemic. Hyperlipidemia significantly lowered the fraction unbound in plasma by approximately 31%. Conclusions. The altered pharmacokinetics of nifedipine by P407-induced HYPERLIPIDEMIA may be, at least in part, due to the decrease in fraction unbound in plasma. A decrease in intrinsic clearance, however, cannot be ruled out.     

Probenecid Inhibits the Metabolic and Renal Clearances of Zidovudine (AZT) in Human Volunteers

The effect of probenecid on the disposition of AZT was investigated in a pilot study in two healthy volunteers. The pharmacokinetics of AZT were examined after a single oral dose of 200 mg with and without probenecid coadministration in a balanced crossover study. Administration of 500 mg probenecid every 6 hr prior to and during AZT dosing resulted in an increase in the average AUC_AZT from 89 µg · min/ml (control) to 191 µg · min/ml during probenecid treatment. This was manifested by a corresponding decrease in CL_TOT/F, which is attributed to the inhibitory effect of probenecid on the glucuronidation and renal excretion of AZT. Average CL_R and CL_TOT/F of AZT decreased from 4.76 and 28.7 to 2.98 and 14.1 ml/min/kg during control and probenecid treatment, respectively. AZT glucuronidation was affected to a greater extent than its renal excretion, as reflected by the decreased ratio of GAZT/AZT urinary recoveries. The terminal half-life of AZT was slightly longer during probenecid administration. That only a small change in the half-life occurred indicates that probenecid also reduced the volume of distribution of AZT. The CL_R of GAZT decreased from an average of 11.3 ml/min/kg (control) to 2.63 ml/min/kg during probenecid treatment, resulting in a greater than 3.5-fold increase in AUC_GAZT. Probenecid did not affect the blood/plasma distribution or the plasma protein binding of AZT. These preliminary findings suggest that it may be possible to maintain effective plasma AZT concentrations in AIDS patients receiving a reduced daily dose, in combination with probenecid.     

In Vivo Assessment of Intestinal,Hepatic, and Pulmonary First Pass Metabolism of Propofol in the Rat

Purpose. The relative contribution of the intestinal mucosa, liver and lung to the in vivo disposition of propofol in the rat was investigated. Methods. Propofol (4.9–5.1 mg · kg^–l) was administered to groups of rats (n = 4) via the intra-arterial, intravenous, hepatic portal venous and oral routes. The AUC's of propofol were estimated and the fractions of the administered dose escaping first pass metabolism by the gut wall (f_G), liver (f_H) and lung (f_L) were calculated. In addition, transport experiments were carried out using Caco-2 cell monolayers to rule out the possibility that intestinal permeability is limiting the oral absorption of propofol. Results. Values for f_G, f_H and f_L were the following: 0.21 ± 0.07, 0.61 ± 0.13, and 0.82 ± 0.09, respectively. The apparent permeability coefficient of propofol across Caco-2 cell monolayers was 24.2 ± 0.3 × 10^–6 cm · sec^–1, which is similar to the apparent permeability coefficient obtained for propranolol (30.7 ± 1.7 × 10^–6 cm · sec^–1), a compound known to easily cross the intestinal epithelial membranes. The formation of propofol glucuronide, a major metabolite of propofol, could not be demonstrated during the flux experiments across the Caco-2 cell monolayers. Conclusions. The intestinal mucosa is the main site of first pass metabolism following oral administration of propofol in the rat. Intestinal metabolism could therefore also contribute to the systemic clearance of propofol.     

Intestinal First-Pass Metabolism of Eperisone in the Rat

Purpose. The purpose of this study was to clarify quantitatively the contribution of the intestine to the first-pass metabolism of eperisone in rats. Methods. The systemic availabilities of eperisone were estimated by administering the drug into the duodenum, portal vein, and femoral vein in rats in vivo. The first-pass metabolism of eperisone was confirmed in the perfused rat small intestine in situ. Metabolism of eperisone to an -1-hydroxylated metabolite (HMO), the first step of eperisone metabolism, was studied using rat intestinal microsomes in vitro. Results. The bioavailabilities in the intestine were 0.176 and 0.0879 at administration rates of 100 and 25 mg/h/kg, respectively, whereas those in the liver were 0.532 and 0.486, respectively. In the intestinal perfusion experiment, the appearance clearance to the portal vein from the intestinal lumen was much lower than the elimination clearance from the intestinal lumen, resulting in high metabolic clearance of eperisone in the small intestine. Eperisone was biotransformed to HMO by rat intestinal microsomes, and this was inhibited by -naphthoflavone and an anti-rat CYP1A antibody. Conclusions. Those data strongly suggest that eperisone may be metabolized to HMO by CYP1A in rat intestinal microsomes during the first-pass through the epithelium of the small intestine.