*5.3. Furosemide*

Rat experiments demonstrated that diabetes decreased plasma furosemide exposure following oral dose but increased plasma furosemide exposure following intravenous dose [105]. Intestinal absorption of furosemide may be involved in MCT6 [84]. It was assumed that intestinal absorption of furosemide was mediated by MCT6. Function of intestinal MCT6 under diabetic status was set to be 50% according previous report [47]. Furosemide is eliminated mainly via renal secretion due to OAT1/2 [3]. Part of furosemide is metabolized by CYP2C11, CYP3As and CYP2E1 [152]. The contribution of CYP2C11 was 61.5% of the total metabolism [152], rests (38.5%) were assumed to attributed to CYP2E1 and CYP3As. Hepatic clearance (CLliver) of furosemide was reported to be 2.20 mL/min/kg [105] and the estimated value of fu × CLint,liver was 0.75 mL/min/250g. Diabetes increased the expression of CYP2E1 and CYP3As by 3-fold [28] and the expression of CYP2C11 mRNA was decreased to 16% that of control rats [47]. Thus, fu × CLint,liver and CLliver were estimated to be 0.75 mL/min/250g and 2.20 mL/min/kg. The estimated CLliver of furosemide was near to observed data (2.98) in diabetic rats [105]. Diabetes altered expression of these targeted proteins (CYP2C11, CYP3As, CYP2E1 and OATs), intestinal blood, liver blood flow and renal blood flow. It was assumed that intestinal absorption of furosemide was mediated by MCT6. All these contribute to the altered furosemide pharmacokinetics, whose integrated effects (Figure 6) were predicted using PBPK model. The results demonstrate that the contribution of increases in CYP3A and CYP2E1 to hepatic clearance of furosemide was partly attenuated by decreases in CYP2C11, which may explain why non-renal clearance of furosemide was only slightly increased under diabetic status [105]. PBPK simulation demonstrated that decreases in renal excretion of furosemide due to downregulation of renal OATs

and renal blood flow rates mainly contributed the increased plasma exposure of furosemide following an intravenous dose. Decreased oral exposure of furosemide was mainly attributed to the impairment of intestinal absorption due to the suppression of intestinal MCT6. The contribution of decreased expression of renal OATs to the altered oral plasma exposure of furosemide was less than that of intestinal MCT6 impairment, whose net effect was to decrease oral plasma of furosemide (Figure 6G).

**Figure 6.** Observed (point) and predicted (line) plasma concentrations of furosemide (Fu) following (**A**) oral (6 mg/rat, ig) and (**B**) intravenous dose (6 mg/rat, iv) to diabetic rats (DM) and control rats (CON). Comparison of mean observed and predicted plasma concentrations of furosemide at each time point (**C**). Contributions of altered (**D**) renal OATs, (**E**) intestinal MCT6 and (**F**) renal blood flow (Qkidney) to oral plasma exposure of furosemide as well as (**G**) integrated effects of intestinal MCT6 impairment and renal OATs impairment. The observations were obtained from the literature [89].
