Gary L. Neil

Acivicin in 1985

This review, as its title indicates, views acivicin at a particular point in the ongoing process of its development. There is a large body of biochemical information which permits the formulation of a number of hypotheses regarding the drugs optimal regimen, mechanism of CNS toxicity, and potential role in combination chemotherapy. We have attempted to survey those data and to project some avenues of future research which may circumvent the drugs limitations. Current deficits exist in our information, particularly in the area of the clinical activity spectrum of acivicin. Yet the final definition of the set of human tumors in which acivicin may find clinical utility will probably not occur until we have defined the optimal regimen for the drug, both as a single agent and in combination, and have identified and addressed the toxic effects which limit its use. A coordinated effort between the preclinical pharmacologists and clinicians will be necessary in the next few years, if acivicin is to play an important role in the treatment of human malignancies.

Immunosuppressive, antiviral and antitumor activities of cytarabine derivatives

Abstract Although cytarabine (cytosine arabinoside, ara -cytidine, Cytosar) is a potent immunosuppressant, antiviral and antitumor agent in animals and man, maximum inhibitory effects require the use of complex injection schedules. Previous reports have shown that good immunosuppressive and antitumor activities were attained with simple injection schedules using the 5′-adamantoate derivative. The current results show that a variety of 5′-acylates were equally as active as the 5′-adamantoate in suppressing immune responses in rodents (hemagglutinin formation in mice and hamsters, skin graft rejection in rats), and as antitumor agents in mice (L1210 leukemia). Similar results were attained in protecting mice from the lethal effects of intracranial herpes simplex infection, and in inhibiting DNA synthesis in phytohemagglutinin-stimulated human lymphocytes. The hypothesis for the enhanced potency of these newer derivatives was as follows: After injection of these insoluble derivatives, there is a finite time required for dispersion and solubilization. The freely circulating derivatives are resistant to deamination (and inactivation). After enzymatic hydrolysis to the free acid and cytarabine, the latter is then free to exert its inhibitory activities. The net effect is the maintenance of relatively low levels of cytarabine for long periods of time.

Studies of the biochemical pharmacology of the fermentation-derived antitumor agent, (αS, 5S) -α-amino-3-chloro-4, 5-dihydro-5-isoxazoleacetic acid (at-125)

Abstract (αS, 5S)-α-Amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (AT-125) isolated from fermentation broths of Streptomyces sviceus was found to have significant activity against a number of tumors in experimental animals. It is currently being developed by the U.S. National Cancer Institute for clinical evaluation. AT-125 was shown to be a potent inhibitor of growth of L1210 and KB cells in culture. These effects were markedly dependent on the time of exposure to the agent. Similar results were observed when survival (residual proliferative capacity) of L1210 cells was measured. With L1210 cells (in culture and in vivo) AT-125 inhibited the incorporation of 3H-TdR into macromolecules to a greater extent than was observed when 3H-UR was the precursor. The activity of AT-125 in crude fermentation broths was detected in an in vitro antimicrobial prescreen designed to specifically detect materials with antimetabolite activity. Its activity in such a system was rather specifically antagonized by the amino acid, L-histidine. Such was not the case, however, in mammalian cells; L-histidine was without effect on AT-125 activity. In mammalian cells, the effects of AT-125 on growth inhibition or inhibition of radioactive precursors into macromolecules were rather specifically antagonized by L-glutamine. AT-125 was found to have significantly greater toxicity towards female than male mice, and sensitivity appeared to decrease with animal age. Further studies showed that AT-125 toxicity could be alleviated by coadministration of testosterone. In this respect, the biological activity of AT-125 was quite similar to that of a nucleoside antitumor agent, deaza UR. The primary locus of activity of deaza UR (after conversion to the triphosphate) has been shown to be inhibition of CTP synthetase. In studies with rat liver homogenates, AT-125 has been shown to inhibit CTP synthetase as well as other enzymes which catalyze the transfer of the amido group of L-glutamine. Further investigation of the effects of AT-125 have shown it to be a very potent inhibitor (Ki = 2 × 10−6 M) of purified rat liver CTP synthetase. AT-125 also causes effects on ribonucleotide pools which are consistent with inhibition of this enzyme and with the inhibition of another L-glutamine-dependent enzyme, XMP aminase. The importance of these two enzymes as loci for the biochemical action of AT-125 was supported when it was found that a combination of CR and GR significantly antagonized the growth inhibitory activity of AT-125.

Biochemical and pharmacologic studies with 1-β-d-arabinofuranosylcytosine 5′-adamantoate (NSC-117614), a depot form of cytarabine

In the treatment of L1210 leukemic mice, the antitumor activity observed with 1-β-d-arabinofuranosylcytosine 5′-adamantoate (AdO-ara-C) suggested that the agent represents a molecular depot form or sustained action form of 1-β-d-arabinofurano-sylcytosine (ara-C, cytarabine). The results of the present study provide strong support for this hypothesis. Many biochemical similarities between ara-C and AdO-ara-C were observed, suggesting similar or identical modes of action. Cytotoxicity of both agents in cell culture could be prevented with deoxycytidine. Like ara-C, AdO-ara-C markedly inhibited DNA synthesis with little effect on RNA or protein synthesis. Cross-resistance (L1210 cells in culture) was also observed. The differences observed in vitro (e.g. lower intrinsic Cytotoxicity, lower extent of uptake by L1210 cells, slower kinetics of inhibition of DNA synthesis with the derivative) were also consistent with the hypothesis that hydrolysis of AdO-ara-C to ara-C is required for cytotoxic activity. Direct evidence for hydrolysis was obtained in studies of the metabolism in vitro of AdO-ara-C in mammalian plasma and in plasma level studies in mice. Inhibition of enzymatic hydrolysis with an esterase inhibitor (eserine sulfate) markedly reduced the Cytotoxicity of AdO-ara-C towards L1210 cells in culture. Plasma level and excretion studies indicated that i.p. administration of AdO-ara-C to mice yielded cytotoxic ara-C levels which persisted for much longer than is possible with a single dose of the parent compound itself. These data, when considered with those concerning the effects of low levels of ara-C in contact with cells in culture for long periods help to explain the unusual therapeutic effects of AdO-ara-C.

A multi-end point in vitro system for detection of new antitumor drugs.

By utilizing new types of producing microorganisms and isolating these on rather unusual growth media, we hope to produce new classes of antitumor drugs. In the detection system, we included the highly sensitive L1210 in vitro assay. But be requiring additional antimicrobial activity, we were able to eliminate rather early most of the previously known drugs from further work-up. The screening protocol was arranged so as to detect antimetabolites of a few rationally selected compounds.

Effectiveness of Antitumor Agents Administered Subcutaneously to L1210 Leukemic Mice in Silicone Rubber Devices

The therapeutic effects of a number of antineoplastic agents administered subcutaneously to L1210 leukemic mice in silicone polymer (Silastic®, Dow Corning Medical Grade) implants is reported. 1-β-d-a