Anatomical and physiological basis for the allometric scaling of cisplatin clearance in dogs.

Cisplatin is a platinum-containing cytotoxic drug indicated for the treatment of solid tumors in veterinary and human patients. Several of the algorithms used to standardize the doses of cytotoxic drugs utilize allometry, or the nonproportional relationships between anatomical and physiological variables, but the underlying basis for these relationships is poorly understood. The objective of this proof of concept study was to determine whether allometric equations explain the relationships between body weight, kidney weight, renal physiology, and clearance of a model, renally cleared anticancer agent in dogs. Postmortem body, kidney, and heart weights were collected from 364 dogs (127 juveniles and 237 adults, including 51 dogs ≥ 8 years of age). Renal physiological and cisplatin pharmacokinetic studies were conducted in ten intact male dogs including two juvenile and eight adult dogs (4-55 kg). Glomerular filtration rate (GFR), effective renal plasma flow, effective renal blood flow, renal cisplatin clearance, and total cisplatin clearance were allometrically related to body weight with powers of 0.75, 0.59, 0.61, 0.71, and 0.70, respectively. The similar values of these diverse mass exponents suggest a common underlying basis for the allometry of kidney size, renal physiology, and renal drug handling.

Pharmacokinetics of valacyclovir in the adult horse.

Recent outbreaks of equine herpes virus type-1 infections have stimulated renewed interest in the use of effective antiherpetic drugs in horses. The purpose of this study was to investigate the pharmacokinetics of valacyclovir (VCV), the prodrug of acyclovir (ACV), in horses. Six adult horses were used in a randomized cross-over design. Treatments consisted of 10 mg/kg ACV infused intravenously, 5 g (7.7-11.7 mg/kg) VCV delivered intragastrically (IG) and 15 g (22.7-34.1 mg/kg) VCV administered IG. Serum samples were obtained at predetermined times for acyclovir assay using high-performance liquid chromatography. Following the administration of 5 g VCV, the mean observed maximum serum ACV concentration (C(max)) was 1.45 +/- 0.38 (SD) microg/mL, at 0.74 +/- 0.43 h. At a dose of 15 g VCV, the mean C(max) was 5.26 +/- 2.82 microg/mL, at 1 +/- 0.27 h. The mean bioavailability of ACV from oral VCV was 60 +/- 12% after 5 g of VCV and 48 +/- 12% after 15 g VCV, and did not differ significantly between dose rates (P > 0.05). Superposition suggested that a loading dose of 27 mg/kg VCV every 8 h for 2 days, followed by a maintenance dose of 18 mg/kg every 12 h, will maintain effective serum ACV concentrations.

Transplacental pharmacokinetics of flecainide in the gravid baboon and fetus.

The objective of this study was to characterize the transfer of flecainide across the placenta and determine the fetal: maternal ratio of flecainide in the gravid baboon. Flecainide acetate has been especially successful for the treatment of fetal supraventricular tachycardia associated with hydrops fetalis. However, the degree of transplacental transmission remains unknown. In this study, all animals were placed under general anesthesia. Flecainide 2.5 mg/kg was administered intravenously. Percutaneous umbilical blood sampling was performed simultaneously with maternal sampling. Flecainide levels were measured using high-performance liquid chromatography with ultraviolet detection. A total of six gravid baboons were studied at an average gestational age of 132 days. The mean maternal volume of distribution at steady state was 5.1 +/- 1.8 L/kg. The mean combined elimination constant (k(el)) was 0.79 +/- 0.19 hr(-1) [95% confidence interval (CI), 0.64-0.93]. There was a linear relationship between maternal and fetal concentrations, with a ratio of fetal-to-maternal serum levels of 0.49 +/- 0.05 (95% CI, 0.39-0.59). At steady state, fetal flecainide levels are approximately 50% of maternal flecainide levels. Flecainide is rapidly distributed in the mother and fetus following a single intravenous dose with a maternal volume of distribution similar to that reported in normal healthy human adults. Since fetal levels correlate closely with maternal levels, we propose that it is possible to estimate fetal levels by monitoring maternal levels.

A liquid chromatographic method for the simultaneous determination of promethazine and three of its metabolites in plasma using electrochemical and UV detectors.

A new assay method has been developed for the quantitation of promethazine (PMZ) with a sensitivity and reproducibility as good as any previously reported method. This method is also capable of quantitatively determining three metabolites of PMZ (monodemethylated, sulphoxidated, and monodemethylated sulphoxidated PMZ), which has not been previously described. The method uses high-performance liquid chromatography with amperometric and UV detection simultaneously and requires only one extraction step from serum with chloroform. The method uses trifluoperazine as the internal standard. The limit of detection level for PMZ is 1.0 ng/ml when a 0.2-mL specimen of plasma is assayed. A validation study is also conducted for evaluating the recovery, precision, linearity of response, sensitivity, and selectivity of the method.

Bioavailability of intranasal promethazine dosage forms in dogs.

Intramuscular promethazine (PMZ) is used aboard the US Space Shuttle to ameliorate symptoms of space motion sickness. Bioavailability after an oral dose of PMZ during space flight is thought to be impaired because of gastrointestinal disturbances associated with weightlessness and space motion sickness. In an attempt to find an alternative dosage form for use in space, we evaluated two intranasal (i.n.) dosage forms of PMZ in dogs for absorption and bioavailability relative to that of an equivalent intramuscular dose. Promethazine (5 mg kg-1) was administered as two intranasal dosage forms and as an intramuscular (i.m.) dose to three dogs in a randomised cross-over design. Serial blood samples were taken and analysed for PMZ concentrations and the absorption and bioavailability of PMZ were calculated for the three dosage forms. PMZ absorption from the carboxymethyl cellulose microsphere i.n. dosage form was more rapid and complete than from the myverol cubic gel formulation or from an i.m. injection. Bioavailability of the microsphere formulation was also greater than that of the gel formulation (AUC 3009 vs 1727 ng h ml-1). The bioavailability of the two i.n. dosage forms (relative to that of the i.m. injection) were 94% (microsphere) and 54% (gel). The i.n. microsphere formulation of PMZ offers great promise as an effective non-invasive alternative for treating space motion sickness due to its rapid absorption and bioavailability equivalent to the i.m. dose.

Using the Internet as a pharmacokinetic resource.

The Internet is an growing resource for scientific information, and includes information which can be useful to the pharmacokineticist. Once connected to the Internet there is a wide variety of information sources available. Collaboration with colleagues, research groups, discussion lists or news groups can be a valuable use of the Internet. Interactive discussions are possible using Internet Relay Chat or other conferencing software. There are a multitude of data, software and other information available via FTP, gopher or World Wide Web servers. This information may be exclusively research-oriented or more useful for instructional use, or suitable for both.

Disposition of aerosolized liposomal amphotericin B.

Amphotericin B (AmB) is an important drug for the treatment of fungal infection, but toxicity limits the lung tissue doses which may be achieved through intravenous administration. Although incorporation of AmB in liposomes reduces these effects and increases the therapeutic index for intravenous administration, targeted delivery to lung tissues via inhaled liposomal AmB aerosol may be a more effective approach. Aerosolization of liposomal amphotericin B targets the lungs, the organs first infested by many fungi. Development of optimal aerosolized liposomal AmB therapies requires a better understanding of the effect that liposome surface charge has on lung clearance kinetics. In this work we evaluated the clearance kinetics and organ distribution of inhaled liposomal AmB in male Balb/C mice. Mice were exposed via nose only to AmB-containing liposomal aerosols having positive, negative, or neutral surface charge characteristics. The formulations were aerosolized using a Collison nebulizer. Groups of animals were euthanized at predetermined times and the lungs and other organs were analyzed for AmB. AmB was not detected in serum and other organs such as kidneys, liver, and brain. The disposition of neutral and positive liposomal amphotericin B in lungs followed biexponential kinetics. The alpha and beta phase half-lives for positive liposomes were 1.3 and 15.1 days, respectively, and 2.3 and 22 days for neutral liposomes. AmB delivered via negative liposomes exhibited monoexponential clearance with a half-life of 4.5 days. These results suggest that toxic side effects in nontarget tissues are minimal and may indicate a potential for long term protection against fungal infections.

Gastrointestinal absorption of phenytoin from an oil-in-water microemulsion.

The absorption profile of phenytoin Na emulsion were examined compared to that of phenytoin suspension after oral administration in the rat. The corn oil-in-water emulsion, particle size of 184+/-57.8 nm, was prepared using a microfludizer, and phenytoin Na added by shaft homogenizer. The phenytoin emulsion or suspension, 100 mg/kg, were intubated intragastrically using oral dosing needle and blood samples were withdrawn via an indwelling cannula from the conscious rat. Plasma concentrations of phenytoin were measured with HPLC using phenacetin as an internal standard. The plasma concentration versus time data were fitted to a one compartment open model and the pharmacokinetic parameters were calculated using the computer program, Boomer. The phenytoin plasma concentrations from the emulsion at each observed time were about 1.5-2 times higher than those from the suspension, significantly at time of 5, 6 and 7 hr after administration. The absorption (k(a)) and elimination rate constant (k(e)) were not altered significantly, however the AUC increased from 65.6 to 106.7 mug.hr/ml after phenytoin suspension or emulsion oral administration, respectively. From an equilibrium dialysis study, the diffusion rate constant (k(IE)) was considerably higher from the phenytoin Na emulsion (0.0439 hr(-1)) than phenytoin suspension (0.0014 hr(-1)).

New high-performance liquid chromatographic method for amphotericin B analysis using an internal standard.

A simple and reproducible HPLC method for the analysis of amphotericin B (AmB) in serum, lung and liver using natamycin as the internal standard was developed. AmB and natamycin were extracted from serum, lung and liver and were separated using an isocratic elution from C18 reversed-phase column. The mobile phase consisted of acetonitrile-10 mM acetate buffer pH 4.0 (37:63, v/v). The HPLC system had two detectors in series. One was set at 303 nm and the other at 383 nm for the detection of natamycin and AmB, respectively. The retention times of AmB and natamycin were 15 and 6 min, respectively. The recovery efficiency was 96%-70%. The limit of quantification was 0.1 microgram/ml. The assay was reproducible, the within-day coefficient of variation (n = 6) was < 8% for serum, lungs and liver. The between-day variability (n = 6) was < 7.7% for serum, liver and lungs at 1 microgram/ml or 1 microgram/g tissue concentration. The assay was linear within the range 1-40 micrograms/ml (r2 = 0.99).

Bioavailability of intranasal scopolamine in normal subjects.

The bioavailability of scopolamine in three dosage forms was compared in 12 healthy nonsmoking male volunteers. Subjects received 0.4-mg doses of scopolamine bromide in intravenous (i.v.), intranasal (i.n.), or oral (p.o.) dosage forms on three occasions, with at least 2 weeks separating the doses. Scopolamine concentrations in plasma were determined with a combined reverse-phase liquid chromatographic-radioreceptor binding assay. Saliva volume and flow rate and percent suppression of control flow rate were determined from each sample. Absorption after i.n. and po scopolamine administration was rapid; plasma concentrations [1680 (i.n.) and 164 pg/mL (p.o.)] peaked within 1 h of dosing [0.37 (i.n.) and 0.78 h (p.o.)], respectively. i.n. and i.v. scopolamine suppressed salivary flow rate to similar extents (95% and 99.7%), respectively. Times to reach maximum effect were 1.05 and 0.27 h after i.n. and i.v. dosage, respectively. Absolute intranasal bioavailability, calculated from the area under the drug concentration vs time curve, was found to be significantly greater than that of p.o. scopolamine (83% vs 3.7%, p < 0.05). The i.n. route may provide a noninvasive, reliable, fast, and effective route for administering scopolamine.

Metabolism and metabolite pharmacokinetics of BRB-I-28, a class Ib antiarrhythmic agent.

The metabolism of BRB-I-28 (7-benzyl-3-thia-7-azabicyclo[3.3.1]nonane), a novel class Ib antiarrhythmic agent, was characterized in vivo in dogs and rats and in vitro with rat liver microsomal preparations containing a NADPH-generating system. In dogs, rats and the in vitro hepatic microsomal oxidation system, BRB-I-28 was extensively metabolized to form 7-benzyl-3-thia-7-azabicyclo [3.3.1]nonane-3-oxide (I), a major metabolite. The metabolite I was produced via S-oxidation, presumably by the hepatic P-450 system. Formation of minor metabolite, 7-benzyl-3-thia-7-azabicyclo[3.3.1]nonane (II) via the oxidation of the benzylic site was also identified in rats. Following intravenous and oral administration of BRB-I-28 to dogs, the plasma concentration of major metabolite I could be test described by a 1-compartmental model. The plasma AUC of metabolite I was 20% (i.v.) and 179.4% (oral) of that of the parent BRB-I-28, respectively, suggesting that BRB-I-28 was metabolized significantly by the first pass effect following oral administration. Extensive metabolism of BRB-I-28 to form metabolites I and II, which have demonstrated much lower antiarrhythmic activities, further supports previously observed pharmacodynamic and pharmacokinetic findings.

Effects of localized Pasteurella haemolytica infection on erythromycin-binding properties of bovine alpha-1-acid glycoprotein, albumin, serum, and tissue chamber fluids.

The in vitro erythromycin-binding properties of bovine alpha-1-acid glycoprotein (AAG) and albumin were studied by using equilibrium dialysis. In addition, the proportions of free erythromycin in bovine serum and tissue chamber fluid before and 4 days after inoculation of subcutaneous tissue chambers with Pasteurella haemolytica were measured. At a concentration of 5 micrograms/ml, erythromycin was moderately bound to AAG (39% +/- 4% free) and was only slightly bound to albumin (86% +/- 2% free). Scatchard analysis of the data describing binding to pure bovine AAG indicated that erythromycin was bound to a single high-affinity (6.45 x 10(4) M-1) site on the protein. At lower total concentrations of erythromycin, the free concentrations of the antibiotic were lower in serum samples collected after infection (49% +/- 3% at 5 micrograms of erythromycin per ml) than in those collected before inoculation (55% +/- 3% at 5 micrograms of erythromycin per ml). Inoculation had no effect on binding to macromolecules in chamber fluids. Inoculated tissue chambers served as a convenient model for studying the effect of infection on drug-macromolecule interactions in interstitial fluid.

Clinical and medicoeconomic impact of the cyclosporine-diltiazem interaction in renal transplant recipients.

The effect of the diltiazem-cyclosporine interaction on cyclosporine pharmacokinetics, pharmacodynamics, and pharmacoeconomics was studied in 10 recipients of renal allografts. Each subject was studied while receiving diltiazem 60 mg twice/day and while not taking the drug. After achieving steady-state conditions, cyclosporine and metabolite concentrations were determined in whole blood from samples drawn after the morning cyclosporine dose. After pharmacokinetic analysis, all patients were followed for 6 months during treatment with cyclosporine plus diltiazem or cyclosporine alone. Cyclosporine blood clearance decreased significantly after treatment with diltiazem (18.0-11.0 ml/min.kg; p = 0.008). The apparent volume of cyclosporine distribution also decreased significantly (4.26-2.62 L/kg; p < 0.05). After 6 months, diltiazem had no effect on renal function indexes, and no apparent effect on immunosuppression. Alterations in cyclosporine clearance and apparent volume of distribution secondary to diltiazem result in dosage reduction and potential cost savings in transplant pharmacotherapy. The mean decrease in cyclosporine dosage requirements would produce a cost saving of $1520 or 28% per patient per year.

Application of physiologically based pharmacokinetic models for assessing drug disposition in space.

Exposure to weightlessness induces physiologic changes that may lead to pharmacokinetic and pharmacodynamic alterations of drugs administered to crew members in flight. Preliminary data from flight and ground-based studies indicate that pharmacologically significant changes occur in the kinetics of medications given in weightlessness and in simulated microgravity (head-down bed rest). Conducting flight studies on all available medications to identify the changes in their pharmacokinetic behavior in weightlessness is not feasible. An alternative approach for obtaining such information is to use computer simulations employing physiologically based pharmacokinetic (PBPK) models. Information thus obtained would be helpful in predicting the therapeutic effectiveness of medications in space, and also in developing plans for flight studies. This paper presents a brief review of relevant physiologic factors and pharmacokinetic implications of space flight, and includes a preliminary PBPK model for estimating plasma concentration-time profiles of acetaminophen under different experimental conditions.

Pharmacokinetic evaluation of a new oral cyclosporine formulation.

STUDY OBJECTIVE: To compare the pharmacokinetics of a new oral cyclosporine preparation with those of cyclosporine solution diluted in Isocal and the intravenous formulation. DESIGN: Randomized, crossover trial. SETTING: Tertiary care referral center. PATIENTS: Seven pediatric liver transplant recipients who were receiving oral cyclosporine as part of their immunosuppressive regimen. All patients completed the study. INTERVENTIONS: Pharmacokinetic studies were performed with the intravenous and oral dosage forms. Patients received one dose of intravenous cyclosporine, and then were randomized to receive their usual oral cyclosporine dose incorporated into a chocolate wafer or mixed with Isocal. After a minimum of 3 days, the alternative preparation was administered. Serial cyclosporine blood samples were collected at predetermined intervals for 12 hours after the third dose for each regimen. Concentrations were determined by high-performance liquid chromatography. The data for the three dosage forms were fit simultaneously with a two-compartment model. MEASUREMENTS AND MAIN RESULTS: No difference was seen in F, ka, Cmax, and tmax between the two oral cyclosporine preparations (p > 0.05). No new rejection episodes occurred during the study period. CONCLUSIONS: We conclude there is no difference in the bioavailability of the oral solution and the chocolate formulation. We believe the new preparation may increase patient compliance and ensure administration of a complete dose compared with the currently marketed solution.

Distribution of intramuscularly administered erythromycin into subcutaneous tissue chambers before and after inoculation with Pasteurella haemolytica.

Distribution of erythromycin into subcutaneous tissue chambers was characterised pharmacokinetically and the effect of Pasteurella haemolytica infection on the extent of penetration was studied. Thermoplastic tissue chambers were implanted subcutaneously in the paralumbar fossae of six calves. Thirty-five days after implantation, the tissue chamber distribution of intramuscularly administered erythromycin (30 mg kg-1) was studied. Chambers were then inoculated with P haemolytica and the tissue chamber pharmacokinetics of erythromycin were again studied. Diffusion of erythromycin into tissue chambers was best described using a two-compartment model with tissue chambers representing a relatively inaccessible compartment. Despite changes in chamber fluid pH, the extent of erythromycin penetration into chambers was not affected by P haemolytica inoculation. Comparison of computer simulated concentration-time curves resulting from different routes of administration revealed that penetration of erythromycin into less accessible sites was more likely to be higher after intravenous administration than after intramuscular administration.

Physiological modeling of disposition of potential tumor-imaging radiopharmaceuticals in tumor-bearing mice.

Radiopharmaceuticals have great potential in the early detection of human tumors. Three potential 99mTc-labeled platinum compounds based on cisplatin have been synthesized and tested in tumored mice. This report presents the analysis of the disposition data obtained after a single intravenous injection with an empirical, physiologically based pharmacokinetic model. The radioactivity of each radiopharmaceutical after administration was measured in blood, urine, and 15 tissues, including tumor. Parameters included in the model were tissue volumes (experimentally determined), tissue blood flows (determined from literature values), tissue:blood extraction ratios (determined by nonlinear least-squares regression with MULTI-FORTE), and clearance terms (also determined by nonlinear least-squares regression). Data were weighted by the reciprocal of the square of the observed values. Good fits to the experimental data were obtained. As expected, the compound producing the best tumor:blood profile (3) also had the highest tumor extraction ratio (6.2 versus 2.0 and 1.3 for 1 and 2, respectively). Total body clearance values for the radioactivity associated with the three compounds 1-3 were calculated to be 0.09, 0.04, and 0.016 mL/min, respectively. Analysis of data with such an empirical, physiologically based model may assist future development of suitable tumor-imaging agents.

Comparison of MS-DOS and Macintosh pharmacokinetic analysis programs using a two-compartment, two-infusion dosing scheme.

Pharmacokinetic values derived by three nonlinear least-squares regression computer programs for sets of serum drug concentration data were compared. The three programs selected for comparison were the MS-DOS-based programs PCNONLIN and ADAPT and the Macintosh program BOOMER. Data on serum recainam hydrochloride concentration in 10 patients given an i.v. loading dose followed by a maintenance infusion were fitted by the appropriate models in each program. Samples had been subjected to reverse-phase isocratic high-performance liquid chromatography with ultraviolet light detection; intraday and interday coefficients of variation were less than 8% over the range of concentrations measured. A two-compartment model was used for all regressions. Each program was given identical initial estimates, and the simplex minimization algorithm of each program was used to fit the model to the data. An identical weighting scheme was used for all three programs. The mean pharmacokinetic values estimated by each program for the 10 data sets were essentially identical. Slightly larger rate constants and smaller volume terms were derived by BOOMER. BOOMER yielded the lowest weighted sum of squares and the highest correlation coefficient. A mean concentration-time plot showed that the programs all produced values that described the data very well. The three computer programs used in this analysis derived essentially the same pharmacokinetic values to describe sets of serum drug concentration data. The BOOMER program provides an acceptable alternative to MS-DOS-based programs for pharmacokinetic analysis.

Biological evaluation of 99mTc labelled cis-platinum containing benzyliminodiacetic acids as potential tumour imaging agents.

The synthesis of three new potential tumour-imaging radiopharmaceuticals in which a cis-platin derivative is attached to benzyl iminodiacetic acid, a ligand capable of forming a stable complex with 99mTc, has previously been reported by us [Awaluddin et al. Appl. Radiat. Isot. 38, 671-674 (1987)]. We have now carried out extensive biodistribution studies on these compounds as well as on two fragments of their structures which do not contain platinum. The results suggest that the presence of platinum is not essential for the tumour-localizing properties of the radiopharmaceuticals.