*5.2. Verapamil*

Verapamil is substrate of CYP3As and P-gp. Rat experiments showed that diabetes increased plasma exposure of verapamil following an oral dose but decreased plasma exposure of verapamil following an intravenous dose [29]. A semi-PBPK model (Figure 4) was successfully developed to simulate the pharmacokinetics of verapamil (Figure 5A,B) in rats using the parameters listed in Tables 1 and 2. Intestinal P-gp in diabetic rats was set to 60% of control rats [82]. The contributions of intestinal/hepatic CYP3A and intestinal P-gp to alterations in oral plasma concentration of verapamil in diabetic rats were investigated. The results demonstrate that increased oral plasma exposure of verapamil was mainly attributed to the impairment of intestinal P-gp (Figure 5E) and the role of intestinal CYP3As was minor (Figure 5D). Furthermore, the contribution of increased expression of hepatic CYP3As to the altered plasma concentration of verapamil following oral dose to rats was less than that of intestinal P-gp impairment, whose net effect was to increase oral plasma exposure of verapamil (Figure 5G).

**Figure 4.** Schematic diagram of semi-physiologically based pharmacokinetic model (PBPK) model describing the pharmacokinetics of verapamil and furosemide in rats. A<sup>i</sup> , Q<sup>i</sup> and V<sup>i</sup> indicate drug amount, blood flow and volume in corresponding compartment, respectively. k<sup>i</sup> , ka,i and kb,i represent the transit rate constant, drug absorption rate constant and efflux rate constant from enterocytes to the gut lumen, respectively. Clint,gwi, Clint,liver and Clint,kidney mean the intrinsic clearance in enterocytes, hepatocytes and kidney, respectively.



<sup>a</sup> Renal blood flow was reduced by 65% of control rats [147].

**Figure 5.** Observed (point) and predicted (line) plasma concentrations of verapamil (Ver) following (**A**) oral (10 mg/kg, ig) and (**B**) intravenous dose (1 mg/kg, iv) to diabetic rats (DM) and control rats (CON); (**C**) comparison of mean observed and predicted plasma concentrations of verapamil at each time point; (**D**) individual contributions of altered hepatic CYP3A, (**E**) intestinal CYP3A and (**F**) intestinal P-gp to oral plasma exposure of verapamil as well as (**G**) their integrated effects. The observations were obtained from the literature [27].


**Table 2.** Pharmacokinetic parameters of verapamil and furosemide for PBPK simulation in diabetic rats (DM) and control rats (CON).

<sup>a</sup> Estimated using the reported data [150]; <sup>b</sup> estimated according to a method [151] reported by Ruark et al. and physicochemical properties of verapamil; <sup>c</sup> function of intestinal MCT6 was set to be 50% that of control rats [47]; d level of intestinal P-gp protein was 60% that of control rats [82]; <sup>e</sup> fu × Clint were estimated using equation fu\*Clint = Q × CL/(Q − CL); <sup>f</sup> contribution of CYP2C11 was 61.5% of the total metabolism [152], rests (38.5%) were assumed to attributed to CYP2E1 and CYP3As. Diabetes increased the expression of CYP2E1 and CYP3As by 3-fold [28] and expression of CYP2C11 mRNA was decreased to 16% that of control rats [47]; <sup>g</sup> function of renal OATs was decreased to 40% that of control rats [115].
