pH-Dependent Functional Activity of P-Glycoprotein in Limiting Intestinal Absorption of Protic Drugs: Kinetic Analysis of Quinidine Efflux In Situ MANTHENA V.S. VARMA, RAMESH PANCHAGNULA Department of Pharmaceutics, National Institute of Pharmaceutical Education and Research (NIPER), Phase X, SAS. Nagar, Punjab 160062, India Received 3 March 2005; revised 29 June 2005; accepted 29 June 2005 Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.20489 ABSTRACT: The purpose of this investigation is to evaluate the quantitative contribu- tion of pH-dependent passive permeability on the functional activity of P-glycoprotein (P-gp) in limiting intestinal absorption of weakly basic drugs, in order to include this effect in prediction models. pH-dependent octanol/buffer partition coefficient, artificial membrane permeability and in situ rat intestinal permeability of quinidine were determined in the physiological pH range of gastrointestinal tract. In situ permeability, as a function of luminal pH, was also determined in the presence of P-gp inhibitor, verapamil (500 mM). Octanol/buffer partition coefficient, transport across artificial membrane, and rat in situ permeability showed high pH-dependency. Absorption quotient (AQ), calculated from in situ permeability to express the functional activity of P- gp, declined with increase in luminal pH or increase in luminal quinidine concentration because of the increased passive permeability or saturation of P-gp. AQ was 0.57  0.02 and 0.41  0.05, while passive permeability was 0.32  0.01  10 4 cm/sec and 0.43  0.02  10 4 cm/sec, in jejunum and ileum, respectively, at pH 7.4. Further, apparent Michaelis – Menten constants (K M , J P-gp,max ) for the quinidine efflux in jejunum indicated that efflux activity was more at luminal pH 4.5 over pH 7.4. K M values for jejunum quinidine efflux at pH 4.5 and pH 7.4 were determined to be 77.63  10.90 and 22.86  5.22 mM, with J P-gp,max values of 1.47  0.08 and 0.62  0.04 nM/cm 2 /sec, respectively. AQ vs passive permeability showed significant relationship indicating dependency of P-gp-mediated efflux on pH-dependent passive permeability, which is dictated by ionization status for a protic or ampholytic drug. In conclusion, an orally administered drug is absorbed from various segments of intestine, which inherit difference in luminal pH, transcellular permeability and P-gp expression. In situ data suggests that pH-dependency and regional variability in passive permeability of protic substrates significantly influence their P-gp-mediated efflux and may have implications on predictions of the in vivo drug absorption. ß 2005 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 94:2632–2643, 2005 Keywords: pH dependency; passive permeability; P-glycoprotein efflux; prediction; oral absorption INTRODUCTION Absorption of drugs from the gastrointestinal tract is determined by fundamental processes, solubility and permeability. 1 Permeability being complex, is still unpredictable determinant and it 2632 JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 94, NO. 12, DECEMBER 2005 Correspondence to: Ramesh Panchagnula (Telephone: 91- 172-214700; Fax: 91 172 214692; E-mail: panchagnula@yahoo.com) Journal of Pharmaceutical Sciences, Vol. 94, 2632–2643 (2005) ß 2005 Wiley-Liss, Inc. and the American Pharmacists Association