Pyrido[2,3-d]pyrimidin-7-one inhibitors of cyclin-dependent kinases

The identification of 8-ethyl-2-phenylamino-8H-pyrido[2, 3-d]pyrimidin-7-one (1) as an inhibitor of Cdk4 led to the initiation of a program to evaluate related pyrido[2, 3-d]pyrimidin-7-ones for inhibition of cyclin-dependent kinases (Cdks). Analysis of more than 60 analogues has identified some clear SAR trends that may be exploited in the design of more potent Cdk inhibitors. The most potent Cdk4 inhibitors reported in this study inhibit Cdk4 with IC(50) = 0.004 microM ([ATP] = 25 microM). X-ray crystallographic analysis of representative compounds bound to the related kinase, Cdk2, reveals that they occupy the ATP binding site. Modest selectivity between Cdks is exhibited by some compounds, and Cdk4-selective inhibitors block pRb(+) cells in the G(1)-phase of the cell division cycle.

Enhanced therapeutic effect of amsalog (CI-921) in combination with cisplatin in vitro and in vivo

CI-921, the 5-methyl-4-carboxamide analog of amsacrine, in combination with cisplatin produced a 6-fold better cell kill in vitro than expected based on additive effects. The combination of CI-921 and cisplatin was subsequently evaluated in three in vivo model systems: intraperitoneally (TP) and intravenously (IV) implanted P388 leukemia, and advanced stage subcutaneously (SC) implanted LC-12 squamous cell carcinoma. All drug treatments were administered IP on an intermittent treatment schedule which was optimal for both agents. Combination therapy was superior to therapy with the best single agent alone, CI-921, in all three model systems. Against IP implanted P388 leukemia, combination therapy produced greater than 8 logs of net tumor cell kill with 60-day survivors (cures). This level of activity was 50-fold greater (1.7 log) than that obtained with CI-921 alone. An IV implant of P388 leukemia was used in a confirmatory study to provide a more rigorous evaluation against disseminated disease. Combination therapy against IV implanted P388 leukemia produced greater than 7.7 logs of net tumor cell kill, which was 630-fold greater (2.8 logs) than that obtained with CI-921 therapy alone. Against advanced stage LC-12 (200-1000 mg tumors at initial treatment), combination therapy improved tumor cell kill by 0.6 log (4-fold) over that obtained with CI-921 therapy alone and also produced greater numbers of 120-day survivors than did single agent therapy with CI-921. The combination of carboplatin and CI-921 was also evaluated against IV implanted P388 leukemia to determine if the enhanced therapeutic effect of CI-921 and cisplatin could be extended to include CI-921 and carboplatin. Combination therapy with CI-921 and carboplatin increased net log tumor cell kill by 0.8 and 1.5 log in two separate tests (6- and 32-fold, respectively) over that obtained with CI-921 therapy alone. The data indicate that combination therapy with CI-921 and platinum containing anticancer agents may have clinical application.

Dual function nitroimidazoles less toxic than RSU 1069: selection of candidate drugs for clinical trial (RB 6145 and/or PD 130908

Following the toxicity and synthetic difficulties encountered with the hypoxic cell radiosensitizer RSU 1069, efforts have focused on development of a superior analogue. Two compounds, RB 6145 and PD 130908, have emerged from this program which overcome the instability and synthetic problems associated with RSU 1069 while retaining favorable biological activity. Both agents show comparable radiosensitizing activity to RSU 1069 following oral or i.p. administration to mice bearing the KHT or RIF-1 tumors. Sensitizing efficiency is about 10 X greater than that observed for misonidazole or etanidazole. Toxicity toward hypoxic tumor cells in vivo is demonstrated by clamping tumors (for 60 min) following administration of PD 130908 or RB 6145. Both are effective hypoxic cytotoxins, but less potent than RSU 1069. Systemic toxicity is substantially reduced following oral drug administration. Further, doses achievable following fractionated drug treatments are sufficiently high to produce significant levels of radiosensitization.

Activity of the pyrazoloacridines against multidrug-resistant tumor cells

A series of 2-aminoalkyl-5-nitropyrazolo [3,4,5-kl]acridines (pyrazoloacridines) were tested in vitro against a panel of multidrug-resistant cell lines comprising Adriamycin-resistant P388 leukemia, B16 melanoma, and mammary adenocarcinoma 16c. This new class of anticancer agents, particularly the 9-substituted methoxy derivatives, exhibited significant activity against all of the lines tested. The degree of cross-resistance to these compounds ranged from zero to 8-fold in the 138-fold Adriamycin-resistant P388/ADR line and was greatly diminished in the B16/ADR and 16c/ADR lines. Selected pyrazoloacridines were subsequently tested in vivo against B16 and B16/ADR cells established as solid tumors from the tissue culture line and shown to retain a significant degree of Adriamycin resistance. Whereas the B16/ADR line exhibited 2 logs less net tumor-cell kill than the B16 parent in response to Adriamycin treatment, the resistant tumor was completely sensitive to the pyrazoloacridines tested and proved in some experiments to be collaterally sensitive. The favorable activity of the pyrazoloacridines against these Adriamycin-resistant tumor lines points to the potential efficacy of these compounds against multidrug-resistant tumors encountered clinically.