PURPOSE

1. STUDY OF THE EFFECT OF THE XCIPIENT CROSSCARMELOSE ON METFORMIN INTESTINAL PERMEABILITY BY AN IN SITU INTESTINAL PERFUSION MODEL IN RATS Aceituno, A. PhD1,2, Dibsi, A., Pharm D1, Gajardo, A., Pharm D.1 & Pezoa, R., PhD21 [Universidad de Valparaíso], 2[Instituto de Salud Pública de Chile] This research work was funded by the University of Valparaiso Research and Development Division and the Public Health Institute of Chile RESULTS The mean calculated permeability values of Metformin in the whole rat intestinal segment showed significant differences at lower initial drug concentrations (50 and 70 g/mL). Under the experimental conditions, the absorption of Metformin sulfate cannot be described as a passive process at these concentrations. Metformin permeability was dependent on the concentration, therefore the absorption process can be described as an apparent first order kinetic process combined with a saturable efflux. The first order kinetics was used to obtain the first order apparent constants. PURPOSE • To investigate on the permeability BCS classification of the drug Metformin and to study the effect of the potential ionic interaction between the excipient sodium Crosscarmellose (a common tablet disintegrant) and Metformin in pharmaceutical formulations by: • an in situ intestinal perfusion model in rats (close loop). • simultaneous perfusion of the drug solution and Crosscarmellose in a whole intestinal segment. • Assessment of the permeability coefficients, calculated with and without the excipient co-infusion of the drug. METHODOLOGY In situ infusion model An in situ absorption model in rats was used to evaluate the intestinal permeability of Metformin, a provisional BCS class III drug. Three drug concentrations were assayed to investigate on the passive or active absorption mechanism of the drug: 50, 70 and 100 g/mL. A whole intestinal segment was isolated in anesthetized Sprague Dawley rats, washed with an electrolyte solution and cannulated at both distal ends. An isotonic buffered solution of Metformin or metformin plus Crosscarmellose was infused into the segment with the help of glass syringes, and the lumen concentration measured by a validated HPLC technique over a period of 30 minutes at 5 minutes intervals. At the end of this period, rats were euthanized following a previously approved protocol. To investigate the effect of co-infusion of the drug and Crosscarmellose, a concentration of the disintegrant was chosen to represent the amount normally present in conventional tablet formulations. The effect on the drug absorption and permeability was evaluated by statistical comparison of the apparent permeability coefficient of Metformin measured when infused with and without dissolved Crosscarmellose. The isolated segment was first washed with 30 mL of a pH 7,4 solution (solution A) with the purpose to eliminate the remnant intestinal content and 30 mL solution B, in order to condition the intestinal mucosa prior to experiments: Solution A Solution B Proportion of Crosscarmellose aded Shown in the next tables are the volumes of Crosscarmellose added to the Metformin solution perfunded, keeping in all cases the drug/excipient ratio close to the one found in conventional formulations of the drug: The apparent absorption rate constant (Eq. 1) was calculated by fitting the lumen drug concentration left versus time data to a first order kinetics process after correcting the concentration for the passive water reabsorption. The intestinal permeability values were calculated considering the relation between ka and Peff (Eq. 2) where R is the radius of the perfused intestinal segment. Equation 1 Equation 2 Figure 4: theoretical ionic interaction between Metformin and Crosscarmellose Metformin predicted and experimental concentrations at different initial drug concentrations are depicted (Figs. 1, 2, 3). The determination coefficients of the first order regression was always higher than 0,95. Results showed that there was a statistical difference between the values of Metformin permeability coefficients when the drug was perfused with or without Croscarmellose. In the presence of Croscarmellose, there was a statistically significant decrease in intestinal permeability at a concentration of 50 and 70 g/mL (5.4 x 10-5 versus 1.9 x 10-5 cm/sec and 2.0 x 10-5 versus 1.5 x 10-5 cm/sec at 70, respectively). No difference in permeability coefficients was found at a concentration of 100 g/mL (table below). Likewise, there was dependence between the concentration of perfused Metformin and the permeability coefficients measured. Log concentration 50 µg/mL metformin solution 50 µg/mL metformin solution + Crosscarmellose time Figure 1: Average Metformin concentrations in the luminal intestinal content versus time after perfusion (50 g/mL) Log concentration 70 µg/mL metformin solution 70 µg/mL metformin solution + Crosscarmellose time Figure 2: Average Metformin concentrations in the luminal intestinal content versus time after perfusion (70 g/mL) CONCLUSIONS • The rat in situ (close loop) absorption model proved to be useful to predict potential interactions between drug and excipients commonly used in tablet formulation • The simultaneous presence of Croscarmellose and Metformin in a formulation might decrease its absorbed fraction and probably the bioavailability of poorly permeable ionic drug. • Crosscarmellose produced a concentration dependant effect on Metformin intestinal permeability that is obscured by the concentration dependant permeability of the drug itself. Log concentration 100 µg/mL metformin soluton 100 µg/mL metformin solution + Crosscarmellose time Figure 3: Average Metformin concentrations in the luminal intestinal content versus time after perfusion (100 g/mL)