Simple bench‐top bioassays involving brine shrimp lethality, Lemna frond proliferation, and the inhibition of crown gall tumours on potato discs, as well as the human tumour cell lines A‐549 lung carcinoma, MCF‐7 breast carcinoma and HT‐29 colon adenocarcinoma, were compared for their accuracy to detect known in vivo (P‐388) active antitumour agents supplied by the National Cancer Institute. The potato disc assay was the best and showed excellent correlation to in vivo activity (p=0.008). The brine shrimp assay (p = 0.033) proved to be superior or equally as accurate as the in vitro human solid tumour cell lines (p = 0.033‐0.334). The Lemna assay (p = 0.708) showed the poorest correlation. The brine shrimp and potato disc assays are suggested as convenient in‐house prescreens to existing cytotoxicity or antitumour assays.

Taxol has shown good in vivo antitumor activity in a number of test systems. The formulation of taxol for antitumor testing has been difficult. Esterification at either C-2' or C-7 resulted in loss of in vitro tubulin assembly activity but not cytotoxicity. These observations suggested that esters at C-2' and/or C-7, which would tend to promote water solubility, might serve as useful prodrugs of taxol. The reaction of taxol with either succinic anhydride or glutaric anhydride in pyridine solution at room temperature gave the crystalline mono 2'-adducts 1b and 1f, respectively. Salts of these acids (1b, 1f, 1i) were formed by the addition of 1 equiv of the corresponding base, followed by evaporation and/or freeze-drying of the solvent(s). The salts had improved antitumor activity as compared to the free acids. The triethanolamine and N-methylglucamine salts showed greatly improved aqueous solubility and were more active than the sodium salts. The glutarate series was preferred because of the higher activity and the higher yields obtained. 2'-Glutaryltaxol (1f) was coupled with 3-(dimethylamino)-1-propylamine, using CDI, to form in excellent yield the amino amide 1o. The hydrochloride salt (1p) showed good solubility and was extremely potent and active. At 10 mg/kg, in the B16 screen, 1p gave a T/C of 352 with 5 out of 10 cures. In the MX-1 breast xenograft assay, this prodrug gave values of -100 at doses of 40 and 20 mg/kg, with all live animals being tumor free.

Over the past 30 years, the National Cancer Institute has been involved in the preclinical and/or clinical evaluation of the majority of those agents approved for the treatment of cancer. Many of the new agents under consideration in the NCI program are either natural products or derivatives of natural product leads, and of critical importance to their development is the issue of drug supply. In responding to the drug supply crisis which emerged with the demonstration of the clinical efficacy of taxol, the NCI has identified several important lessons for those interested in natural product drug discovery and development. As a result, the NCI has developed plans to avert similar supply crisis in the future by initiating exploratory research projects for large-scale production of promising agents at the earliest possible point following the demonstration of confirmed antitumor activity. These plans, together with a review of the development of taxol, are presented in this paper.

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