Patrick J. Sinko

ESTIMATING HUMAN ORAL FRACTION DOSE ABSORBED: A CORRELATION USING RAT INTESTINAL MEMBRANE PERMEABILITY FOR PASSIVE AND CARRIER – MEDIATED COMPOUNDS

Based on a simple tube model for drug absorption, the key parameters controlling drug absorption are shown to be the dimensionless effective permeability, Peff*, and the Graetz number, Gz, when metabolism or solubility/dissolution is not rate controlling. Estimating the Graetz number in humans and assuming that Paq* is not rate controlling gives the following equation for fraction dose absorbed: F = 1− e−2P*w. The correlation between fraction dose absorbed in humans and Pw* determined from steady-state perfused rat intestinal segments gives an excellent correlation. It is of particular significance that the correlation includes drugs that are absorbed by passive and carrier-mediated processes. This indicates that Pw* is one of the key variables controlling oral drug absorption and that the correlation may be useful for estimating oral drug absorption in humans regardless of the mechanism of absorption.

Predicting fraction dose absorbed in humans using a macroscopic mass balance approach.

A theoretical approach for estimating fraction dose absorbed in humans has been developed based on a macroscopic mass balance that incorporates membrane permeability and solubility considerations. The macroscopic mass balance approach (MMBA) is a flow model approach that utilizes fundamental mass transfer theory for estimating the extent of absorption for passively as well as nonpassively absorbed drugs. The mass balance on a tube with steady input and a wall flux of Jw = PwCb results in the following expression for fraction dose absorbed, F: F = 2 An ∫01C*bdz* where the absorption number, An = L/R · Pw/vz>;, L and R are the intestinal length and radius, Pw is the unbiased drug wall permeability, 〈vz〉 is the axial fluid velocity, C*b = Cb/Co and is the dimension-less bulk or lumen drug concentration, Cb and Co are the bulk and initial drug concentrations, respectively, and z* is the fractional intestinal length and is equal to z/L. Three theoretical cases are considered: (I) Co ≤ S, Cm ≤ S, (II) Co > S, Cm ≤ S, and (III) Co > S, Cm > S, where S is the drug solubility and Cm is the outlet drug concentration. Solving the general steady-state mass balance result for fraction dose absorbed using the mixing tank (MT) and complete radial mixing (CRM) models results in the expressions for the fraction dose absorbed in humans. Two previously published empirical correlations for estimating fraction dose absorbed in humans are discussed and shown to follow as special cases of this theoretical approach. The MMBA is also applied to amoxicillin, a commonly prescribed orally absorbed β-lactam antibiotic for several doses. The parameters used in the correlation were determined from in situ or in vitro experiments along with a calculated system scaling parameter. The fraction dose absorbed calculated using the MMBA is compared to human amoxicillin pharmacokinetic results from the literature with initial doses approximated to be both above and below its solubility. The results of the MMBA correlation are discussed with respect to the nonpassive absorption mechanism and solubility limitation of amoxicillin. The MMBA is shown to be a fundamental, theoretically based model for estimating fraction dose absorbed in humans from in situ and in vitro parameters from which previously published empirical correlations follow as special cases.

Characterization of the Oral Absorption of β-Lactam Antibiotics. I. Cephalosporins: Determination of Intrinsic Membrane Absorption Parameters in the Rat Intestine In Situ

The oral absorption of five cephalosporin antibiotics, cefaclor, cefadroxil, cefatrizine, cephalexin, and cephradine, has been studied using a single-pass intestinal perfusion technique in rats. Intrinsic membrane absorption parameters, “unbiased” by the presence of an aqueous permeability (diffusion or stagnant layer), have been calculated utilizing a boundary layer mathematical model. The resultant intrinsic membrane absorption parameters are consistent with a significant carrier-mediated, Michaelis-Menten-type kinetic mechanism and a small passive component in the jejunum. Cefaclor colon permeability is low and does not exhibit concentration dependent behavior. The measured carrier parameters (±SD) for the jejunal perfusions are as follows: cefaclor, Jmax* = 21.3 (±4.0), Km = 16.1 (±3.6), Pm* = 0, and Pc*= 1.32 (±0.07); cefadroxil, Jmax* = 8.4 (±0.8), Km = 5.9 (±0.8), Pm* = 0, and Pc* = 1.43 (±0.10); cephalexin, Jmax* = 9.1 (±1.2), Km = 7.2 (±1.2), Pm* = 0, and Pc* = 1.30 (±0.10); cefatrizine, Jmax* = 0.73 (±0.19), Km = 0.58 (±0.17), Pm* = 0.17 (±0.03), and Pc* = 1.25 (±0.10); and cephradine, Jmax* = 1.57 (±0.84), Km = 1.48 (±0.75), Pm* = 0.25 (±0.07), and Pc* = 1.06 (±0.08). The colon absorption parameter for cefaclor is Pm* = 0.36 (±0.06, where Jmax* (mM) is the maximal flux, Km (mM) is the Michaelis constant, Pm* is the passive membrane permeability, and Pc*is the carrier permeability. Aminocephalosporin perfusion results indicate that jejunal absorption in the rat occurs by a nonpassive process, with some of the compounds possessing a small but statistically significant passive component, while the colon permeability is low and follows a simple passive absorption mechanism.

Membrane permeability parameters for some amino acids and β-lactam antibiotics: Application of the boundary layer approach

The boundary layer approach to analyzing the results of the perfused intestinal segment method of measuring membrane permeabilities is applied to the amino acids; leucine, valine, phenylalanine, lysine and aspartic acid and the beta-lactam antibiotics, cephalexin and penicillin V. The analysis indicates that in determining the membrane parameters, Pw vs. Cw data are preferable to using Jss = PwCw vs. Cw data. It is further shown that the carrier permeability, Pc* = Jmax*/Km, may be the most significant parameter to consider since luminal amino acid or drug concentration may generally be below the Km value. A comparison of P*c values for the beta-lactams with results for passively absorbed compounds indicates that the cephalosporins would be expected to be well absorbed orally based on the perfusion results. This suggests that this approach may be useful in estimating oral drug absorption for compounds that are absorbed passively as well as by a carrier-mediated mechanism.

Mass balance approaches for estimating the intestinal absorption and metabolism of peptides and analogues: theoretical development and applications

A theoretical analysis for estimating the extent of intestinal peptide and peptide analogue absorption was developed on the basis of a mass balance approach that incorporates convection, permeability, and reaction. The macroscopic mass balance analysis (MMBA) was extended to include chemical and enzymatic degradation. A microscopic mass balance analysis, a numerical approach, was also developed and the results compared to the MMBA. The mass balance equations for the fraction of a drug absorbed and reacted in the tube were derived from the general steady state mass balance in a tube: dM/dZ = {[(2/R)(Pw + kr)]CVL}/vz, where M is mass, z is the length of the tube, R is the tube radius, Pw is the intestinal wall permeability, kr is the reaction rate constant, C is the concentration of drug in the volume element over which the mass balance is taken, VL is the volume of the tube, and vz is the axial velocity of drug. The theory was first applied to the oral absorption of two tripeptide analogues, cefaclor (CCL) and cefatrizine (CZN), which degrade and dimerize in the intestine. Simulations using the mass balance equations, the experimental absorption parameters, and the literature stability rate constants yielded a mean estimated extent of CCL (250-mg dose) and CZN (1000-mg dose) absorption of 89 and 51%, respectively, which was similar to the mean extent of absorption reported in humans (90 and 50%). It was proposed previously that 15% of the CCL dose spontaneously degraded systemically; however, our simulations suggest that significant CCL degradation occurs (8 to 17%) presystemically in the intestinal lumen. Insulin (Mr = 5700), which is metabolized in the intestine primarily by α-chymotrypsin, was chosen for the second application of theory. The simulations show that the intestinal absorption of insulin is approximately 1% of the administered dose. Further, the extent of insulin oral absorption may not exceed 2% even if effective enzyme inhibitors are dosed concurrently since simulations show that insulin absorption is permeability limited. The steady-state macroscopic and microscopic simulation results were comparable and, for the antibiotics, were similar to published clinical results. Therefore, both approaches are useful for estimating the extent of oral peptide absorption and intestinal reaction from in vitro and in situ results.

Carrier mediated transport of amino acids, small peptides, and their drug analogs

Abstract The oral absorption of amino acids, small peptides and their analogs has been studied in rats using single pass intestinal perfusion. The experimental data was analyzed using a modified boundary layer method permitting the calculation of intrinsic membrane absorption parameters. Cefatrizine, l -Dopa, α-methyldopa, and l -PHE demonstrated concentration dependent absorption. Furthermore, the absorption of the amino acid analog, l -Dopa, was significantly inhibited by l -Leucine. Cefatrizine absorption was significantly inhibited by the peptide PHE-PHE (20 mM whereas the amino acid l -PHE (100 mM) did not inhibit its absorption. The results clearly demonstrate that l -Dopa and α-methyldopa oral absorption occurs via the amino acid pathway, while cefatrizine absorption occurs via the peptide pathway. These results indicate that nutrient pathways may function as a possible route for the oral delivery of selected drugs.

Characterization of the oral absorption of several aminopenicillins: determination of intrinsic membrane absorption parameters in the rat intestine in situ

The absorption mechanism of several penicillins was characterized using in situ single-pass intestinal perfusion in the rat. The intrinsic membrane parameters were determined using a modified boundary layer model (fitted value +/- S.E.): Jmax* = 11.78 +/- 1.88 mM, Km = 15.80 +/- 2.92 mM, Pm* = 0, Pc* = 0.75 +/- 0.04 for ampicillin; Jmax* = 0.044 +/- 0.018 mM, Km = 0.058 +/- 0.026 mM, Pm* = 0.558 +/- 0.051, Pc* = 0.757 +/- 0.088 for amoxicillin; and Jmax* = 16.30 +/- 3.40 mM, Km = 14.00 +/- 3.30 mM, Pm* = 0, Pc* = 1.14 +/- 0.05 for cyclacillin. All of the aminopenicillins studied demonstrated saturable absorption kinetics as indicated by their concentration-dependent wall permeabilities. Inhibition studies were performed to confirm the existence of a nonpassive absorption mechanism. The intrinsic wall permeability (Pw*) of 0.01 mM ampicillin was significantly lowered by 1 mM amoxicillin and the Pw* of 0.01 mM amoxicillin was reduced by 2 mM cephradine consistent with competitive inhibition.

Predicting Oral Drug Absorption in Humans: A Macroscopic Mass Balance Approach for Passive and Carrier-Mediated Compounds

There are several models for estimating drug absorption in humans [1–8]. Both physicochemical properties of the drug and physiological/biochemical properties of the gastrointestinal tract affect the extent and/or rate of oral drug absorption. Some of these factors include: pKa, solubility and dissolution rate, aqueous diffusivity, partition coefficient, chemical and enzymatic stability, intestinal pH, transit time, gastrointestinal motility, endogenous substances such as bile salts, and exogenous substances such as nutrients (food). The systemic availability can be further reduced by first-pass hepatic metabolism. Consequently, prediction of absorption is semi-quantitative.