Jacqueline Plowman

Human Tumor Xenograft Models in NCI Drug Development

The methods used by the National Cancer Institute (NCI) for in vivo preclinical development of anticancer drugs were described in detail in the first edition of this book (1). In addition, a series of review articles have charted the evolution of the overall NCI drug discovery process, which began in 1955 (2–12). Although the methodologies associated with xenograft model testing have remained fundamentally the same, during the past 10 yr a series of improvements to preclinical drug testing to expedite in vivo drug development have been made that now precede the employment of xenograft models in the in vivo drug development process. These specialized assays are described in Chapter 8. For the sake of completeness, the present chapter provides (1) a brief history of the in vivo screens used by the NCI, (2) a description of the human tumor xenograft systems that are employed in preclinical drug development, and (3) a discussion of how these xenograft models are employed for both initial efficacy testing as well as detailed drug evaluations.

Enhanced melphalan cytotoxicity in human ovarian cancer in vitro and in tumor-bearing nude mice by buthionine sulfoximine depletion of glutathione.

The development of acquired resistance to alkylating agents frequently limits the effectiveness of chemotherapy in the treatment of ovarian cancer. While the resistance to alkylating agents is multifactorial, the association of drug resistance with elevations in glutathione (GSH) is of potential clinical relevance since there exist pharmacologic means to lower intracellular GSH levels. We have used in vitro and in vivo models of human ovarian cancer to demonstrate that selective inhibition of GSH synthesis with L-buthionine-S,R-sulfoximine (L-BSO) leads to a lowering of GSH levels and an increase in cytotoxicity of the alkylating agent melphalan. In the human ovarian cancer cell line NIH:OVCAR-3, derived from a patient clinically refractory to alkylating agents, L-BSO resulted in a 3.6-fold enhancement of melphalan cytotoxicity. This cell line was also adapted for intraperitoneal growth in athymic nude mice. In this in vivo model, in which the mice die of massive ascites and intraabdominal carcinomatosis, L-BSO given orally in drinking water for 5 days decreased GSH levels in the tumor cells by 96% compared to a 79 and 86% reduction in GSH levels in the bone marrow and gastrointestinal mucosa respectively. Lowering of GSH levels with BSO was not accompanied by an increase in lethality for melphalan in non-tumored nude mice. However, in tumor-bearing nude mice, a single melphalan (5 mg/kg) treatment following GSH depletion with L-BSO was markedly superior to treatment with melphalan alone, producing a 72% increase in median survival time. Furthermore, L-BSO treatment of human bone marrow cells prior to melphalan exposure had little effect on melphalan toxicity as assessed in a CFUc-GM assay. These results suggest that treatment with the GSH synthesis inhibitor BSO may preferentially enhance the cytotoxic effects of alkylating agents against human ovarian cancer and overcome acquired resistance.

Antitumor 2,3-dihydro-2-(aryl)-4(1H)-quinazolinone derivatives: Interactions with tubulin

A series of derivatives of 2,3-dihydro-2-(aryl)-4(1H)-quinazolinone (DHQZ) with known antitumor activity was re-evaluated in the National Cancer Institute cancer cell line screen. Analysis by the COMPARE algorithm suggested that their cytotoxicity derived from interactions with tubulin. Significant inhibition of tubulin assembly and of the binding of radiolabeled colchicine to tubulin was demonstrated with several of the compounds, particularly NSC 145669, 175635, and 175636. The DHQZ derivatives are structurally analogous to a number of antimitotic agents, flavonols and derivatives of 2-styrylquinazolin-4(3H)-one and of 2-phenyl-4-quinolone. Structure-activity analogies between these agents, the combretastatins, and the colchicinoids were analyzed and summarized.

Activity of flavone acetic acid (NSC-347512) against solid tumors of mice

Flavone acetic acid (FAA) is a new antitumor agent that has recently entered Phase I clinical trials. In preclinical studies, we have found that FAA was broadly active against a variety of transplantable solid tumors of mice (colon #51, #07, #10, #26; pancreatic ductal adenocarcinomas #02 and #03; mammary adenocarcinoma #16/C/Adr; M5076 reticulum cell sarcoma and Glasgows osteosarcoma). FAA was curative for colon adenocarcinoma # 10 and pancreatic ductal adenocarcinoma # 03. Thus, for the first time an agent has been identified with very broad, perhaps nearly universal solid tumor activity. FAA was also found to be orally active and stable in solution at 37 °C for 48 h. FAA was selectively cytotoxic in vitro for solid tumors over leukemias L1210 and P388 (in a soft-agar colony formation assay), thus correlating cellular selectivity in vitro with in vivo antitumor activity. The finding that FAA was active in vitro, established that the agent did not need metabolism (activation) outside the tumor cell. The main drawback of FAA was an unusual ‘threshold’ behavior in which only a narrow range of doses were active and splitting the dose markedly decreased activity.

Current NCI Preclinical Antitumor Screening in Vivo: Results of Tumor Panel Screening, 1976–1982, and Future Directions

Publisher Summary This chapter discusses the current National Cancer Institute (NCI) preclinical antitumor screening in vivo. Preclinical models used by the NCI to select new drugs for clinical trial have changed periodically over the years. Changes in screening methodology have resulted from fundamental studies of biologic factors that affect the success of treatment, such as the relationship of tumor cell growth kinetics to drug responsiveness and from retrospective analyses of correlations between clinical and preclinical efficacy. The increased level of effort required to test a compound in a panel of tumors meant that the input of agents for evaluation would have to be limited. Of a number of alternatives considered, it was determined that the most reasonable approach would be to subject materials to a relatively sensitive and cost efficient in vivo prescreens-those showing a modest but reproducible.

Echinomycin: The first bifunctional intercalating agent in clinical trials

SummaryEchinomycin is a quinoxaline antibiotic that was originally isolated from Streptomyces echinatus. Based on its antitumor activity against two i.p. implanted murine tumors, the B16 melanoma, and the P388 leukemia, it was brought into clinical trials by the National Cancer Institute. Recent studies on its cytotoxic action have related its antitumor activity with its ability to bifunctionally intercalate with double stranded DNA.Toxicologic studies were carried out in CDF1 mice and beagle dogs using intravenous injections. For the mice studies the dose ranges were 288–692 mcg/kg (864–2076 mcg/m2) by single bolus, and 112–254 mcg/kg/day (336–762 mcg/m2/day) for five consecutive days. In the dog, dose ranges studied were 8.9–89.4 mcg/kg (178–1788 mcg/m2) by single bolus, and 3.4–33.5 mcg/kg/day (68–670 mcg/m2/day) for five consecutive days. The major toxic effects were found in the gastrointestinal, hepatic, and lymphoreticular systems. These were reversible at all but the highest dose, in dogs that had been treated for five consecutive days.Phase I clinical trials using various intravenous schedules were sponsored by the National Cancer Institute. Nausea, vomiting, reversible liver enzyme abnormalities, and allergic reactions were the most common toxicities encountered. Based on results from these studies, the National Cancer Institute has recently begun phase II trials in a broad range of diseases. These trials will further characterize echinomycins toxic effects and its antitumor activity.

Menogaril: a new anthracycline agent entering clinical trials

SummaryMenogaril [menogarol, 7(R)-O-methymogarol, 7-OMEN] is a new anthracycline agent which was chosen for clinical trials based on:a)broad spectrum activity against a panel of murine tumorsb)lower cardiotoxicity than doxorubicin in the chronic rabbit modelc)differences in biochemical effects from other anthracyclines suggesting a possible difference in mechanism of actiond)murine antitumor activity by oral as well as parenteral routes. Biochemical studies indicated that, in comparison to doxorubicin, menogaril is bound weakly to DNA, inhibits RNA synthesis less, and has different cell cycle phase-specific cytotoxicity. Pharmacology studies in the mouse and dog using HPLC analytical methodology have shown multiexponential clearance from plasma and metabolism of menogaril to a material which co-chromatographs with N-demethylmenogaril in addition to at least two other metabolites of unknown structure. Oral bioavailability studies in the mouse showed significant absorption of menogaril from the gastrointestinal tract followed by first-pass metabolism. In acute toxicity studies in the rat, the dog, and the monkey, dose-related myelosuppression and gastrointestinal toxicity predominated. Phase I clinical trails on menogaril are currently in progress on a variety of schedules.

Flavone acetic acid (LM 975, NSC 347512) A novel antitumor agent

SummaryFlavone acetic acid (FAA) is a synthetic flavonoid compound which has recently begun clinical trials as an antitumor agent based on its striking activity in solid tumor model systems. The pharmacologic behavior of FAA in animals appears to be predictive of both its cytotoxic efficacy and its toxicity to normal tissues (principally the central nervous system and gastrointestinal tract). The design and conduct of phase I studies in man are based upon these principles, with the goal of maximizing their safety and efficacy.

Trimetrexate: a new antifol entering clinical trials

SummaryTrimetrexate, a 2,4-diaminoquinazoline derivative, is a new antifol recently introduced into clinical trials. It differs from methotrexate principally in its transport (not carrier-mediated), and its intracellular retention (not polyglutamylated). Trimetrexate is active against tumors which are methotrexate-resistant on the basis of impaired transport, and has a broader range of antitumor activity in preclinical models. Animal studies predict toxicity principally to the central nervous system, gastrointestinal tract and bone marrow.

Tiazofurin: A new antitumor agent

SummaryTiazofurin is an interesting drug now entering Phase I trials, with marked preclinical antitumor activity against P388 and L1210 leukemias, and the Lewis lung carcinoma. Schedule dependency favoring frequent administration has been noted.The drug has a novel mechanism of action, being metabolized to an inhibitory cofactor of inosine monophosphate dehydrogenase.Tiazofurin is widely distributed after i.v. administration exhibiting a triphasic pattern of plasma decay, with a terminal half-life of 3–16 h in the three species studied. Approximately 90% of the drug was excreted unchanged in the urine within 24 h. A significant potential for the slower release of intracellularly retained drug exists.Anticipated organ toxicities based on the studies described include myelotoxicity, hepatotoxicity and nephrotoxicity. These were mild and reversible at lower doses, and were not seen at levels corresponding to the starting doses in man. A potential for hyperuricemia exists; this should be easily controllable by the use of allopurinol, without compromising the drugs antitumor effect.Phase I trials under the sponsorship of the NCI are underway in a number of institutions.