General method for assessing bioavailability of drugs undergoing reversible metabolism in a linear system

The ratio of first to zeroth moments of the plasma profile of an oral drug undergoing first-pass and linear reversible metabolism

Based on the convolution integral, equations have been derived for the ratio of the first to the zeroth moments of the plasma concentration-time curve (AUMC/AUC) parameters for a drug (p) undergoing first-pass and reversible metabolism and its reversible metabolite (m). According to these equations, the AUMC/AUC of a drug administered orally and of its reversible metabolite can be related to the mean absorption times, the ratios of the fraction of the dose entering the systemic circulation to the bioavailability, the first-time fractional conversion of each compound, and the AUMC/AUC ratios after intravenous administration of each compound. The proposed approach allows a more generalized derivation method for AUMC/AUC of a drug administered orally and undergoing first-pass and reversible metabolism. It is also applicable to any other extravascular route.

Deconvolution method for assessing the absorption of a drug with reversible metabolic pathways

A method has been established to determine the input rate for a drug with reversible metabolic processes. This method is based on linear system properties without imposing a compartment model for describing the disposition of the drug and the metabolite and their interconnections. The solution is a deconvolution method in N-dimensional space. A general solution has been obtained for calculating the parameters needed to describe the input function. The specific solution for a staircase input function (point-area deconvolution) is explicitly derived. Using a staircase input function is not a robust method and can give negative input values when applied to simulated data sets with a large amount of variability. This suggests the need for more robust functional forms to describe the input function.

Determination of bioavailability and systemically available fractions of drugs undergoing reversible metabolism: application to 4-amino-5-chloro-2-[2-(methylsulfinyl)ethoxy]-N-[2- (diethylamino)ethyl]benzamide and its sulfide and sulfone metabolites in rats

Methods are discussed which permit the calculation of the bioavailability (F) and fraction of an oral dose entering the central circulation (f) of a drug and its interconversion metabolite. The interrelationships between the F and f and between the F and systemically available fractions afforded by reversible metabolism are also derived and described. The application of these principles is illustrated by the pharmacokinetic analysis of 4-amino-5-chloro-2-[2-(methylsulfinyl)ethoxy]-N-[2- (diethylamino)ethyl]benzamide (ML-1035, 1) and its sulfide (2) and sulfone (3) metabolites in rats. Like intravenous ML-1035, ML-1035 administered orally underwent metabolic interconversion with 2 but not with 3 in this species. Both ML-1035 and 2 were absorbed rapidly and are pharmacologically active. On average, 8.3 and 13% of an oral dose (152.4 mumol/kg) of ML-1035 were bioavailable as ML-1035 and its sulfide metabolite, respectively, while 23 and 65% of a molar equivalent dose of the sulfide metabolite were bioavailable as either compound, respectively. Thus, the sulfide metabolite is better absorbed than ML-1035 in rats. Following oral administration of either ML-1035 or 2, the systemically available fractions of both compounds were weakly to moderately influenced by the reversible metabolism process in rats. Moreover, the bioavailability of the sulfone metabolite was very poor (2.5-4%) following separate oral administration of ML-1035, 2, and 3.

A multidimensional deconvolution method for evaluating the absorption of drugs undergoing reversible metabolism and presystemic metabolite formation

A general multidimensional deconvolution method is developed primarily for application to the bioavailability assessment of drugs with reversible metabolism. The proposed approach involves fitting continuous functions to the experimental data and then applying a numerical deconvolution algorithm to the fitted curves. It is shown how the method may be used to determine the rates and extends of drug absorption and presystemic formation of interconversion metabolites. The proposed method is implemented as a FORTRAN subroutine called DCVSYS for the general multidimensional deconvolution problem involving any continuous functions and as a user-friendly PC-compatible program called NDCREV for evaluating the absorption of drugs undergoing reversible metabolism. The methods and computer programs are demonstrated by application to simulated drug and metabolite concentrations for a drug with reversible metabolism.

A method for calculating the mean absorption time of drugs undergoing reversible and first-pass metabolism

A method has been derived for calculating the mean absorption time of an oral drug and its interconversion metabolite which is generated from the drug systemically and presystemically. The method evolves from the convolution integral and requires plasma AUC and AUMC values after separate intravenous administration of the drug and its interconversion metabolite and oral administration of the drug. It can also be used to calculate the mean input time of a drug undergoing reversible metabolism and administered by any other extravascular route. Results of a simulation study using both errorless and errant data indicate that, when the absorption rate constant of a drug or its interconversion metabolite is not much larger than the apparent elimination rate constant, the proposed method performs satisfactorily. However, when the absorption rate constant of a drug or its interconversion metabolite is much larger than the apparent elimination rate constant, the proposed method appears to be inaccurate.

Mean interconversion times and distribution rate parameters for drugs undergoing reversible metabolism

The mean interconversion time and recycling numbers are introduced as intrinsic metabolic interconversion and distribution parameters for drugs undergoing linear reversible metabolism. Equations for these parameters, the distribution clearance, and the mean transit time in the central and peripheral compartments are derived for a metabolic pair where interconversion and elimination occur in central compartments. These parameters can be calculated from plasma concentration versus time slopes and intercepts, AUC, and AUMC data of parent drug and its metabolite partner following iv administration of each compound. The mean time analysis is illustrated with disposition data obtained previously for methylprednisolone and methylprednisone in the rabbit. Examination of mean times and additional properties of the system reveals that total exposure time of methylprednisolone is weakly influenced by the metabolic interconversion process, whereas the total exposure time of methylprednisone is strongly influenced by the process. In addition, the tissue distribution process moderately influence the total exposure times of both compounds. The derived mean time parameters, along with previously evolved equations for clearances, volumes of distribution, moments, and mean residence times allow comprehensive analysis of linear, multicompartmental reversible metabolic systems.

An integrated approach to pharmacokinetic analysis for linear mammillary systems in which input and exit may occur in/from any compartment

The general treatment of linear mammillary models employing input and disposition functions and Laplace transforms is expanded to solve concentration-time equations AUC and AUMC in any compartment without restricting sites of input or output. In this integrated approach to noncompartmental pharmacokinetic analysis, the values of AUC and AUMC can be calculated from Laplace transforms with some very simplified treatments. Tables of input functions, disposition functions, Laplace transforms, and derivatives of the Laplace transforms are presented. The relationships between the integrated parameters and various mammillary model parameters are presented using several examples.

Relative and absolute bioavailability of prednisone and prednisolone after separate oral and intravenous doses

A randomized, four-way cross-over study was conducted in eight healthy male volunteers to determine the relative and absolute bioavailability of prednisone (PN) and prednisolone (PL). PN and PL were administered as single, oral 10-mg tablet doses and as 10-mg zero-order 0.5-hour intravenous infusions. Comparable mean PN and PL maximum plasma concentrations (Cmax), times for Cmax, areas under the plasma concentration-time curves (AUC), and apparent elimination rate constants between tablet treatments demonstrated that PN and PL tablets were bioequivalent. Absolute bioavailability (F) determinations based on plasma PL concentrations were independent of which IV treatment was used as reference and indicated complete systemic availability of PL from both PN and PL tablets. However, F based on plasma PN data was contradictory. Using IV PN as reference, approximately 70% systemic availability was observed from both tablets, whereas using IV PL as reference, systemic availability was greater than unity. PN and PL are model compounds that exemplify the difficulties involved in accurately determining the relative and absolute bioavailability of substances that undergo reversible metabolism.