Continuous five-day infusion of PALA and 5FU: a pilot phase II trial

Current treatment modalities in advanced colorectal carcinoma

Several biochemical and immunological substances are currently being tested for their efficacy in the treatment of advanced colorectal cancer, the second-most malignant disease in the western world. Presently, a combination of 5-fluorouracil (FU) and the biochemical modulator folinic acid is considered to be the standard treatment for this malignancy, with a median response rate of 30%. A recent multi-analysis of phase III trials with methotrexate and FU versus FU alone has demonstrated a statistically significant doubling of the response rate as well as a significant survival advantage for the combination. So far, other biochemical or immunological modulators of FU have not shown a significant advantage regarding response or survival. Several new drugs such as tegafur, thymidilate synthase inhibitors, trimetrexate, or topoisomerase-I-inhibitors have been tested with promising results in preclinical and early clinical settings. While local treatment such as hepatic artery infusion has become safer, it is still more toxic than systemic treatment and showed a significant improvement of response but no survival advantage. Since all treatment strategies in advanced colorectal cancer are still palliative, quality of life is a more important endpoint in clinical trials.

The role of low-dose PALA in biochemical modulation

Phosphonacetyl-L-aspartate (PALA) is a rationally-synthesized analog of the transition-state intermediate in the formation of carbamyl aspartate from carbamyl phosphate and aspartic acid by aspartate carbamyl transferase (ACTase). PALA is thus a potent inhibitor of the enzyme (Ki about 10(-8) M for ACTases of various origins), which in whole cells blocks the de novo synthesis of pyrimidines. In vivo, low doses of PALA inhibit whole body pyrimidine synthesis. While this action is cytotoxic in vitro, extensive human testing demonstrates that PALA alone is devoid of selective antitumor activity. Recent interest in the therapeutic action of PALA derives from the demonstration that its action potentiates the cytotoxicity of several cytotoxic drugs, notably 5-fluorouracil (5-FU). Results from clinical trials of PALA and 5-FU in combination in colorectal cancer suggest that biochemical modulation with regimens which follow the principles determined in preclinical studies may enhance the efficacy of current therapy.

Metabolism and action of amino acid analog anti-cancer agents

The preclinical pharmacology, antitumor activity and toxicity of seven of the more important amino acid analogs, with antineoplastic activity, is discussed in this review. Three of these compounds are antagonists of L-glutamine: acivicin, DON and azaserine; and two are analogs of L-aspartic acid: PALA and L-alanosine. All five of these antimetabolites interrupt cellular nucleotide synthesis and thereby halt the formation of DNA and/or RNA in the tumor cell. The remaining two compounds, buthionine sulfoximine and difluoromethylornithine, are inhibitors of glutathione and polyamine synthesis, respectively, with limited intrinsic antitumor activity; however, because of their powerful biochemical actions and their low systemic toxicities, they are being evaluated as chemotherapeutic adjuncts to or modulators of other more toxic antineoplastic agents.

Cancer chemotherapy. Progress and expectations, 1984

Progress in the treatment of cancer with drugs has radically altered the clinical approach to patients with malignancy. Not only have new drugs produced promising results in hitherto untreatable tumors, but they have extended and enhanced the effectiveness of other modalities, including surgery and radiotherapy. In this article, the authors consider avenues of research that likely to aid in the discovery of new anticancer drugs and improve the effectiveness of established agents. Promising new efforts in drug development include the use of new screening systems, particularly those employing human tumor material; the development of improved analogs of existing active agents particularly those of the anthracycline and platinum complex types; and the search for agents that which promote differentiation or prevent metastasis. In an effort to improve the effectiveness of established agents, the authors consider the application of pharmacokinetic principles in developing regional perfusion routes, intraperitoneal chemotherapy, and central nervous system penetration. Finally, the contribution of biochemical pharmacology to the current understanding of drug action, mechanisms of resistance, and drug interactions are considered, and the impact of this knowledge on clinical protocol design is assessed.