Influence of exposure and infusion times on the cytotoxicity and pharmacokinetics of cis-malonato[(4R, 5R)-4,5-bis(aminomethyl)-2-isopropyl- 1,3-dioxolane]platinum(II)

The effect of exposure time on the in vitro cytotoxicity of a new platinum complex, cis-malonato-[(4R,5R)-4,5-bis(aminomethyl)-2-isopropyl- 1,3-dioxolane]platinum(II) (SKI 2053R) and cisplatin (CDDP) toward two human lung-adenocarcinoma cell lines (PC-9, PC-14) and two human stomach- adenocarcinoma cell lines (KATO III, MKN-45) was investigated by variation of the exposure time (1, 4, 12, and 24 h) and drug concentration to yield a constant product of drug concentration times exposure time (C x T). Exposure of cancer cells to low concentrations of SKI 2053R for 12 or 24 h resulted in a greater killing effect than did1- or 4-h exposure to 24- or 6-fold higher concentrations; the inhibitory effects of SKI 2053R on the colony formation of all tumor cell lines except for KATO III were significantly increased with increasing exposure time (P < 0.05). However, the inhibitory effects of CDDP against all tumor cell lines tested except for PC-14 were inversely correlated with increasing exposure time (P < 0.05). The intracellular accumulation of SKI 2053R and CDDP was measured under the same conditions used in the cell-survival assay using MKN-45 cells. The amount of platinum accumulated from SKI 2053R into MKN-45 cells was greater for the treatment involving low concentrations and long-term exposures (12 and 24 h) than for that using high concentrations and short-term exposures (1 and 4 h) at the constant C x T values; however, the increased accumulation of CDDP was more prominent as the concentration was increased, even if the exposure time became shorter. The pharmacokinetics studies of SKI 2053R following 1-, 4-, 12-, and 24-h infusions were performed in beagle dogs. A single dose of SKI 2053R (5.0 mg/kg) was successively given over various infusion periods to three beagle dogs at 3-week intervals. The peak levels of ultrafiltrable platinum observed for SKI 2053R at the 1-, 4-, 12-, and 24-h infusions were 3.10 ± 0.49 (mean ± SD), 1.24 ± 0.06, 0.43 ± 0.07, and 0.25 ± 0.04 μg/ml, respectively. The mean binding ratios of platinum from SKI 2053R to plasma protein at the end of 1-, 4-, 12-, and 24-h infusions were approximately 91%, 73%, 53%, and 51%, respectively. The steady-state level of free platinum was maintained during long-term infusions (12 and 24 h) after short periods (1-3 h) from the start of the infusion. This study strongly suggests that the therapeutic efficacy of SKI 2053R given by continuous long- term infusion should be investigated in future clinical studies.