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Bladder Wall Penetration of Intravesical Mitomycin C in Dogs¹

Division of Urology [M. G. W., R. A. B., J. R. D.] and College of Pharmacy [J. T. D., J. L-S. A.], The Ohio State University, Columbus, Ohio 43210-1228

We examined the kinetics of penetration of mitomycin C (MMC) in the dog bladder wall after intravesical instillation of 20 mg/40 ml, a dose used in patients. Bladder tissues were harvested and concentration-depth profiles were established by analysis of thin tissue slices cut parallel to the urothelial surface of the bladder. Tissue concentrations after a dwell time of 5–7 min were similar to those after 30–120 min. In tissues harvested 60 and 75 min after removal of the dose, MMC was not detected in 5 of 6 samples and was <1 µg/g at the mucosa in the remaining sample, suggesting a rapid "washout" of the drug. The rapid equilibrium between the drug in urine and bladder tissue indicates that the duration of exposure of the bladder wall tissue was approximately equal to the dwell time of intravesical therapy. Tissue concentrations declined loglinearly with respect to the depth of penetration. The concentration immediately underneath the urothelium (C₀) showed considerable intra- and interanimal variability. Bladder distention appeared to increase C₀ by several fold. C₀ ranged from 2 to 275 µg/g wet tissue weight, with a median value of 24 µg/g, or 11 µg/g when two animals with distended bladders were excluded. MMC concentrations in 3 different sites of the same bladder varied up to 5-fold. Within the capillary-perfused mucosa and muscularis (between 50 and 2000 µm from the urothelial surface), concentrations decreased by 50% for each 500-µm distance. The median concentration at 2000 µm was 1 µg/g (n = 24). At 2000–3000 µm, tissue concentrations in most (18 of 24) specimens either declined to an asymptotic value or were lower than the detection limit of 0.1 µg/g. Concentrations in the bladder contents were 200–500 µg/ml, the average tissue concentration from 50 to 3000 µm was 10 µg/g, and plasma concentrations were <0.1 µg/ml. This supports the therapeutic advantage of intravesical therapy of high local drug concentrations while minimizing systemic exposure. A comparison of the urine concentration and C₀ indicated a 30-fold decline in concentration across the urothelium. This suggests the importance of the urothelium as a barrier to MMC absorption. A separate study in our laboratories showed that 16 µg/ml of MMC was needed to produce a 90% inhibition of the labeling index of explants of human bladder cancers located in the urothelium (T_a tumor, TNM classification), 25 µg/ml in the lamina propria (T₁ tumors), and 43 µg/ml in the muscle layer (T₂ tumors). A comparison of these cytotoxic concentrations with the MMC concentration-depth profiles showed that effective concentrations were achieved in the urothelium in all cases and in the lamina propria in 20% of cases. Concentrations in the muscle layers were <20% of the 90% inhibition concentration for T₂ tumors. These data indicate that the barrier function of the urothelium and intravesical dwell time are major determinants of tissue concentrations and exposure. The variable MMC pharmacokinetics in the bladder tissue layers may contribute to the variable and incomplete patient response to intravesical therapy. Furthermore, cytotoxic concentrations were more readily achieved in the superficial than in the deeper tissue layers. This is consistent with the more favorable response of superficial tumors (T_a) compared to invasive tumors (T₁ and T₂) observed clinically.

¹ This work was partly supported by Research Grant RO1 Ca-49816 and a Research Career Development Award for J. L-S. Au (K04 CA-01497) from the National Cancer Institute, Department of Health and Human Services, and by the University Urology Research Foundation. J. T. D. was supported in part by a fellowship from the American Foundation for Pharmaceutical Education.

² To whom requests for reprints should be addressed, at Division of Urology, The Ohio State University, 456 West 10th Avenue, Columbus, OH 43210-1228.

Received 3/21/91. Accepted 6/ 3/91.

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