Novel Sterically Hindered Platinum Complex: In Vivo Activity, Toxicology, and Pharmacokinetics in Mice’

A novel sterically hindered platinum complex, AMD473 [cis-amminedichboro(2-methylpyridine) platinum(II)J, designed primarily to be less susceptible to inactivation by thiobs, has shown in vitro activity against several ovarian carcinoma cell lines. Notably, AMD473 has shown activity in vitro in human carcinoma cells that have acquired cisplatin resistance due to reduced drug transport (41M/4lMcisR) or enhanced DNA repair/increased tolerance of platinum-DNA adducts (CH1/CHlcisR). In this study, we show that AMD473, at its maximum tolerated dose of 35-40 mg/kg i.p. administration, produced marked in vivo antitumor activity against a variety of murine (ADJIPC6 plasmacytoma, L1210 leukemia) and human ovarian carcinoma xenograft models, including several possessing acquired resistance to cisplatin [ADJ/PC6cisR, Ll2lOcisR, CHlcisR, and HX11O (carboplatin-resistant)]. In the ADJIPC6 model, an increased therapeutic index was noted following oral as opposed to i.p. administration. In a head-to-head comparison using CHlcisR xenografts and equitoxic doses (q7d x4 schedule), comparative growth delays were as follows: AMD473, 34 days; cisplatin, 10.4 days; carboplatin, 6.4 days; and JM216 (p.o. administration), 3.5 days (in a previous experiment, the trans-platinum complex JM335 induced a growth delay of 5.4 days against this model). In this model, oral activity was also noted with a growth delay of 34 days at 400 mg/kg every Received 4/10/97; revised 7/I 1/97: accepted 7/1 1/97. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. I This study was supported by grants to the Institute of Cancer Research from the Cancer Research Campaign and the Medical Research Council. 2 To whom requests for reprints should be addressed, at Cancer Research Campaign Centre for Cancer Therapeutics, The Institute of Cancer Research, 15 Cotswold Road, Sutton, Surrey, SM2 5NG, United Kingdom. Phone: 44-181-643-8901; Fax: 44-181-770-7885: E-mail: [email protected]. 7 days (total of four doses). In addition, AMD473 showed promising activity against CH1 xenografts that had regrown following initial treatment with cisplatin (additional growth delay of 30 days over that observed for retreatment with cisplatin). Across the whole panel of cisplatin-sensitive to cisplatin-resistant human ovarian carcinoma xenografts, AMD473 showed improved or at least comparable activity to that observed for an equitoxic dose (4 mg/kg) and schedule of cisplatin. Platinum pharmacokinetics showed that following i.v. administration of 20 mg/kg AMD473 in saline to Balb/c mice bearing murine plasmacytoma (ADJIPC6), a biexponential decay was observed in the plasma with a rapid distribution tl/2a of 24 mm followed by a slow elimination ti,213 of 44 h. Platinum accumulated in various organs with platinum tissue to plasma area under the curve ratios of 8.6 for liver and kidney, 5.7 for spleen, 3.7 for heart, 5.2 for lung, and 5 for tumor. The plasma and tissue concentration time curve following i.p. administration was similar to that observed following i.v. administration, with a bioavailability of 89%. When AMD473 was given p.o., the platinum absorption was rapid (K01 of 30 mm) and the bioavailability was 40%. A less than proportional increase in area under the curve and Cmax was noted in tissue, plasma, and plasma ultrafiltrate following increasing oral doses of AMD473. In vitro, with AMD473, the rate of binding to different plasma proteins was approximately half of that of cisplatin. Following administration of 45 mg/kg i.p. in oil, 33% of the administered platinum was eliminated in the urine after 24 h, and 40% was eliminated after 72 h. Fecal recovery represented 13% of the administered dose after 3 days. Similar results were observed following oral and i.v. administration of 20 mg/kg, but significantly more was excreted in the feces (over 50% of the administered dose) following oral administration of 400 mgfkg, showing that absorption might be a limiting factor by this route of administration. The doselimiting toxicity for AMD473 in mice was myebosuppression, and no renal toxicity was observed. The promising antitumor activity of AMD473, together with its lack of nephrotoxicity and favorable pharmacokinetic profile, suggests that AMD473 is a good candidate for clinical development. AMD473 is entering Phase I clinical trials under the auspices of the United Kingdom Cancer Research Campaign in 1997.