As preclinical chemotherapists we are often asked to identify experimental tumor models that can accurately predict for the drug response characteristics of all tumors of a given cellular subtype or molecular target Unfortunately it is impossible to give satisfactory answers to these inquiries Because of the unique character of each independently arising tumor (whether spontaneous or induced) it does not take very long to realize that each tumor is a unique biologic entity with its own tumor growth behavior histological appearance drug response and molecular expression profiles This is true whether the tumor is an experimental animal model or one originally derived from a patient Further many factors can influence the tumor growth and therapy response of experimental tumor models Still in vivo models are needed to adequately assess pharmacodynamics toxicity and efficacy of any potential novel therapy Presented herein is what we hope will be useful information regarding the transplant characteristics of tumor models with some of the pitfalls to look out for when using any given tumor model for chemotherapy evaluations Although most of the examples given use syngeneic models the methodologies tor assessing the predictive worth and maintaining model usefulness can be applied to almost any given transplantable tumor system (whether syngeneic or xenograft)

A highly active and broadly active thioxanthone has been identified: N-[[1-[[2-(Diethylamino)ethyl]amino]-7-methoxy-9-oxo-9H-thioxanthen-4-yl] methylformamide (SR271425, BCN326862, WIN71425). In preclinical testing against a variety of subcutaneously growing solid tumors, the following %T/C and Log10 tumor cell kill (LK) values were obtained: Panc-03 T/C = 0, 5/5 cures; Colon-38 (adv. stage) T/C = 0, 3/5 cures, 4.9 LK; Mam-16/C T/C = 0, 3.5 LK; Mam-17/0 T/C = 0, 2.8 LK; Colon-26 T/C = 0, 1/5 cures, 3.2 LK; Colon-51 T/C= 0, 2.7 LK; Panc-02 T/C = 0, 3.1 LK; B16 Melanoma T/C = 13%, 4.0 LK; Squamous Lung-LC12 (adv. stage) T/C = 14%, 4.9 LK; BG-1 human ovarian T/C = 16%, 1.3 LK; WSU-Br1 human breast T/C = 25%, 0.8 LK. The agent was modestly active against doxorubicin (Adr)-resistant solid tumors: Mam-17/Adr T/C =23%, 0.8 LK; and Mam-16/C/Adr T/C = 25%, 1.0 LK, but retained substantial activity against a taxol-resistant tumor: Mam-16/C/taxol T/C = 3%, 2.4 LK. SR271425 was highly active against IV implanted leukemias, L1210 6.3 LK and AML1498 5.3 LK. The agent was equally active both by the IV and oral routes of administration, although requiring approximately 30% higher dose by the oral route. Based on its preclinical antitumor profile, it may be appropriate to evaluate SR271425 in clinical trials.

Two new series of antitumor agents, 4-aminomethylthioxanthenones (6-50) and 5-aminomethylbenzothiopyranoindazoles (51-61), are described and compared. Nearly all members of both series display excellent in vivo activity versus murine pancreatic adenocarcinoma 03 (Panc03) although there is little to distinguish the two series from each other. In both series there is no discernible relationship between structure and in vivo efficacy. Selected analogues were evaluated in vitro; all were observed to have moderate to strong DNA binding via intercalation. However, varying degrees of in vitro P388 cytotoxicity and topoisomerase II inhibition were seen. In general, those molecules which exhibited strong topoisomerase II inhibition were significantly more cytotoxic than those which did not. In both series, those derivatives (48-50, 60, and 61) having a phenolic hydroxy substitution exhibited the most potent P388 cytotoxicity and topoisomerase II inhibition.

PURPOSE:Tirapazamine (3-amino-1,2,4-benzotriazine 1,4-dioxide; SR 4233) is a bioreductive agent that exhibits relatively selective cytotoxicity towards cells under hypoxic conditions and can enhance the antitumor activity of many standard oncolytics. In the present study we examined the interaction between tirapazamine in vivo with paclitaxel and paraplatin in two- and three-way combination studies using the MV-522 human lung carcinoma xenograft model.METHODS:Agents were administered as a single i.p. bolus, with tirapazamine being given 3 h prior to paclitaxel, paraplatin, or their combination. Tumor growth inhibition (TGI), final tumor weights, partial and complete responses, and time to tumor doubling were determined after drug administration.RESULTS:Tirapazamine as a single agent was ineffective against this human lung tumor model. A substantial increase in TGI was seen in animals treated with the triple-agent regimen (tirapazamine-paclitaxel-paraplatin) compared to animals treated with double-agent regimens that did not include tirapazamine. The addition of tirapazamine to paclitaxel-paraplatin therapy resulted in a 50% complete response rate; there were no complete responses seen when only the paclitaxel-paraplatin combination was administered. Time to tumor doubling was also significantly improved with the addition of tirapazamine to the paclitaxel and paraplatin combinations. Tirapazamine did not increase the toxicity of paclitaxel, paraplatin, or their combinations as judged by its minimal impact on body weight and the fact that no toxic deaths were observed with tirapazamine-containing regimens.CONCLUSIONS:These results are important since recent studies have suggested that the combination of paclitaxel and paraplatin may be particularly active in patients with advanced stage non-small-cell lung cancer. Since tirapazamine can significantly improve efficacy, but does not appear to enhance the toxicity of paclitaxel and paraplatin, its evaluation in future clinical trials in combination with paclitaxel-paraplatin-based therapy appears warranted.

Thioxanthones are aromatic hydrocarbons with cytotoxic activity against several tumor models. Potential mechanisms of action may include DNA intercalation, inhibition of nucleic acid biosynthesis, and topoisomerase inhibition, as well as formation of intracellular DNA single strand breaks. Such a broad spectrum of expected antitumor activity makes this class of compounds particularly interesting and worth pursuing in clinical studies. SW 33377 (Win 33377, SR 233377) was so promising in vitro that it was taken into Phase I clinical trials for further evaluation. The compound had undesirable cardiac effects, so new analogs were sought that would have similar antitumor effects without the undesirable side effects. In the present study, two new analogs SW 68210 (WIN 68210), and SW 71425 (WIN 71425) are compared to the antiproliferative action of SW 33377 against a variety of freshly explanted human tumor specimens using an in vitro soft agar cloning system. All compounds were more effective with continuous exposure than 1 hour exposure and a concentration-response effect was evident with all compounds. SW 68210 with continuous exposure showed similar activity to SW 33377 at all concentrations. SW 71425 with continuous exposure was less effective at the lower concentrations but was nearly as effective at 10 μg/ml as the other two compounds and was highly effective at 50 μg/ml. At the 10 μg/ml concentration all compounds were similarly effective against breast, colon, non-small cell lung, and ovarian tumors. The two new analogs, SW 68210 and SW 71425 have activity similar to SW 33377 and are both likely candidates for further development.

Historically, many new anticancer agents were first detected in a prescreen; usually consisting of a molecular/biochemical target or a cellular cytotoxicity assay. The agent then progressed to in vivo evaluation against transplanted human or mouse tumors. If the investigator had a large drug supply and ample resources, multiple tests were possible, with variations in tumor models, tumor and drug routes, dose-decrements, dose-schedules, number of groups, etc. However, in most large programs involving several hundred in vivo tests yearly, resource limitations and drug supply limitations have usually dictated a single trial. Under such restrictive conditions, we have implemented a flexible in vivo testing protocol. With this strategy, the tumor model is dictated by in vitro cellular sensitivity; drug route by water solubility (with water soluble agents injected intravenously); dosage decrement by drug supply, dose-schedule by toxicities encountered, etc. In this flexible design, many treatment parameters can be changed during the course of treatment (e.g., dose and schedule). The discovery of two active agents are presented (Cryptophycin-1, and Thioxanthone BCN 183577). Both were discovered by the intravenous route of administration. Both would have been missed if they were tested intraperitoneally, the usual drug route used in discovery protocols. It is also likely that they would have been missed with an easy to execute fixed protocol design, even if injected IV.

Novel derivatives of SR 233377 (1, WIN 33377) where a pyrazolo ring fusion has been incorporated at the 1- and 9-positions of the thioxanthone ring displayed outstanding in vivo efficacy against the murine solid tumor Panc 03 (T/C values of 0% with log cell kill ⩾ 2.0). No relationship between structure and Panc 03 activity was observed because all analogues studied were highly active.

(S)-10-(2,6-Dimethyl-4-pyridinyl)-9-fluoro-3-methyl-7-oxo-2,3-dihydro-7H-pyrido[1,2,3de][1,4] benzothiazine-6-carboxylic acid (WIN 58161) is an enantiomerically pure quinolone with outstanding bacterial topoisomerase II (DNA gyrase, EC 5.99.1.3) inhibitory and antibacterial activity. Unlike most quinolones, WIN 58161 also exhibits significant inhibitory activity against mammalian topoisomerase II (EC 5.99.1.3). DNA gyrase and topoisomerase II inhibitory activities are enantioselective. Consequently, WIN 58161 and its enantiomer (WIN 58161-2) provide useful tools to probe the contribution of topoisomerase II inhibition to the mechanism of cytotoxicity of quinolones and the potential utility of quinolone-topoisomerase II inhibitors as antitumor agents. WIN 58161 inhibited both highly purified Escherichia coli DNA gyrase and HeLa cell topoisomerase II by the promotion of enzyme-DNA covalent complexes. WIN 58161 did not bind stably to DNA via intercalation and did not enhance the formation of topoisomerase I (EC 5.99.1.2)-DNA covalent complexes. At drug concentrations that are cytotoxic to P388 murine leukemia cells, WIN 58161 promoted intracellular DNA single-strand breaks (SSBs) that exhibited the hallmarks of being mediated by topoisomerase. DNA fragments were complexed with protein, and SSBs were readily resealed at 37 ° following drug removal. WIN 58161-2 was neither cytotoxic nor did it promote intracellular SSBs in P388. These observations suggest that the mechanism of cytotoxicity of WIN 58161 is predominantly, if not exclusively, a result of topoisomerase II inhibition. When studied in tumor-bearing mice, WIN 58161 exhibited a significant antitumor effect against each of five tumors tested, whereas neither toxicity nor antitumor activity was observed with WIN 58161-2. We conclude from these studies that WIN 58161 represents the prototype of a novel chemical class of topoisomerase II inhibitor with potential clinical utility in treating cancer.

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Mammalian topoisomerase II inhibition activity has been identified in a series of novel pyrazoloquinoline derivatives; potency for two analogues containing cyclohexyl groups at the 2-position was comparable to the reference agents, mAMSA and VP-16. In several instances, topo II inhibition translated to a high level of in vitro cytotoxicity and murine antitumor activity.

Highly efficacious and curative murine anti-solid tumor activity has been identified in a novel series of 4-aminomethylthioxanthones. One of these new agents, 1 (WIN 33377), is in the preclinical stage of development.

WIN33377 (Sterling/Kodak) entered Phase I clinical trials in 1994. The dose limiting toxicity has not been reached with completion of the 225 mg/m2 level, Q28 day schedule. To date, the agent has been very well tolerated with no evidence of liver toxicity. Eventually, a weekly schedule will be undertaken, which is consistent with the rapid host recovery time for this agent (six days). WIN33377 is an analogue of Hycanthone, an antischistosomal agent, that also has antitumour activity in preclinical models. However, Hycanthone is very poorly tolerated at the efficacious dose levels. Clinical trials of Hycanthone were carried out between 1978 and 1983, producing severe liver toxicity with drug induced deaths. No antitumour activity was recorded. WIN33377 and a variety of analogues were discovered to have markedly improved antitumour activity, and were well tolerated in mice. Most analogues had no evidence of liver toxicity with WIN33377 being totally devoid of liver toxicity. The key to better efficacy and t...

Several enantiomerically pure (2,6-dimethyl-4-pyridinyl)quinolones, previously shown to be potent inhibitors of bacterial DNA gyrase, exhibit topoisomerase II inhibitory activity. Among these and other analogues, topoisomerase II inhibitory potency was found to be a sensitive function of the size and substitution of the bridge spanning the 1- and 8-positions of the quinoline ring. The 6-fluoro group was required for activity.

1-Cyclopropyl-6,8-difluoro-1,4-dihydro-7-(2,6-dimethyl-4-pyridinyl)-4-ox o-3-quinolinecarboxylic acid (1), a previously reported potent inhibitor of bacterial DNA gyrase, was found to be interactive with mammalian topoisomerase II (topo II). In a DNA-cleavage assay using topo II isolated from HeLa cells, 1 exhibited an EC50 value of 7.6 microM (VP-16; EC50 = 0.81 microM). A series of analogues modified at the 1-, 2-, 3-, 5-, and 7-positions of 1 were subsequently made and assessed for topo II inhibition. Compound 1 was considerably more potent than derivatives where the 1-substituent was alkyl, aryl, or H, or when N-c-C3H5 was replaced with S. The descarboxyl (i.e., 3-H) analogue had potency comparable to that of 1; when both these compounds were substituted at the 2-position with methyl or phenyl, an interesting relationship between activity and the conformation of the carboxyl group emerged. Upon replacement of the 5-H of 1 with NH2 or F, sustained potency was seen. No enhancement of activity was evident upon replacing the 7-substituent of 1 with other pyridinyl groups, 4-methyl-1-piperazinyl, or pyrrolidinyl groups; however, the 7-(4-hydroxyphenyl) analogue (CP-115,953) was 6-fold more potent than 1. The topo II inhibitory properties of 1 translated to modest in vitro cytotoxicity and in vivo activity versus P388.

The history of antitumor drug discovery has essentially been the use of two lymphocytic leukemias of mice as selection funnels through which all agents needed to pass in order to advance toward clinical development (L1210 prior to 1975 and P388 after 1975). It is thus not surprising that agents in the clinic are highly active against these tumor systems. However, none of the agents discovered by these leukemias are tumor specific (i.e., active against all tumors), and none of the agents are broadly active against solid tumors of either rodents or humans (1–3). An example contrasting the responsiveness of transplantable solid tumors of mice and the two leukemias is shown in Table-1. The lack of responsiveness of these solid tumors of mice is not unlike those seen in human lung, pancreatic, colon, and prostate tumors. The point to emphasize is that the lack of solid tumor activity of available antitumor agents is not species related. The fault does not lie with the omission of human tumors in the initial selection process, but rather with the omission of solid tumors.