Biodistribution and pharmacokinetics of colon-specific HPMA copolymer–9-aminocamptothecin conjugate in mice

Abstract

A water soluble N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer -9-aminocamptothecin (9-AC) conjugate was designed for oral colon-specific drug delivery for the treatment of colon cancer. Comparative studies between the polymer conjugate and free drug have been performed to assess their biodistribution and pharmacokinetics in mice. After oral administration of equal doses of the polymer conjugate or free 9-AC, the drug concentrations in major organs at fixed time points were determined using an HPLC-fluorescence assay. Only 2±1% of 9-AC released from the polymer conjugate was detected in small intestine (SI), and the mean peak concentration of free 9-AC was 45-fold higher than that from released drug. Colon-specific release of 9-AC produced high local concentrations. The mean peak concentration of released 9-AC in cecal contents, feces, cecal tissue, and colon tissue were, respectively, 3.2-fold, 3.5-fold, 2.2-fold and 1.6-fold higher than that using free 9-AC. In plasma, the high and sharp drug concentration profile from free drug was in contrast to the relatively low and flat pharmacokinetic profile obtained from drug released from the HPMA copolymer. There was no significant difference between released and free drug for the area under the concentration-time curve (AUC) and bioavailability values. As a consequence of the colonspecific release of unmodified 9-AC from the polymer conjugate, antitumor efficacy can be anticipated to be enhanced due to prolonged colon tumor exposure to higher and more localized drug concentrations.

Pharmacokinetic Modeling of Absorption Behavior of 9-Aminocamptothecin (9-AC) Released from Colon-specific HPMA Copolymer–9-AC Conjugate in Rats

“…Previously, we reported the metabolism of 9-AC in liver of mice and identified one liver metabolite produced by hydroxylation of the amino group of 9-AC (14). To quantitatively determine the drug hepatic availability (F h ) in rats (n = 4), liver perfusion was performed.…”

“…The absorbed fraction of 9-AC was calculated as 0.63±0.14 based on the difference between the starting and remaining concentration in the cecum. It was reported that 9-AC was metabolized via the cecal wall after absorption from the cecum (14). The cecal availability (F c ) of 9-AC was calculated as 0.35±0.12 according to Eq.…”

“…Based on our previous data, less than 3% of the HPMA copolymer-9-AC conjugate was degraded in the small intestine (SI). We assumed that 97% of the polymer conjugate reached the cecum (13,14). Orally administered polymer conjugate is transited from the stomach and SI into the cecum, where it is degraded to release free 9-AC.…”

“…To improve local therapy of colon cancer, maximize the therapeutic response, and minimize toxic effects, a drug delivery system for oral colon-specific delivery of 9-AC was designed, with 9-AC attached to an HPMA copolymer carrier via a spacer containing an aromatic azo bond and a self-eliminating 4-aminobenzylcarbamate group (13,14). When the HPMA copolymer-9-AC conjugate reaches the colon, unmodified 9-AC is released from the polymer by the cleavage of the azo bond by the azo-reductase activities of colonic microflora, followed by 1,6-elimination.…”

“…Furthermore, due to slow transit and drug absorption in colon (15), it is expected that colon tumors may be exposed to high 9-AC concentrations for prolonged time with low systemic toxicity. Previously, the biodistribution and pharmacokinetics of this conjugate were studied in mice (14). After oral administration of the HPMA copolymer-9-AC conjugate, a vast majority of 9-AC was released in cecum.…”