Predictive model for plasma concentration-versus-time profiles of investigational anticancer drugs in patients.

We report a model that provides a strong correlation between mouse toxicity data [mouse lethal dose 10% (LD10)] and human plasma concentration-versus-time (CXT) data for 22 commonly used anticancer agents. Mouse toxicity data (LD10) from two dosing schedules, daily times one and daily times seven, were evaluated for the two mouse strains BDF/1 and Swiss. Data from BDF/1 mice were selected for analysis because they were more abundant. Strong correlations were found between LD10 and human plasma CXT data for both daily times one and daily times seven dosing schedules--ln (CXT) = -1.6504 + [0.8408 X ln (LD10)], r = .84, P less than .0001, and ln (CXT) = -0.0754 + [0.8954 X ln (LD10)], r = .90, P less than .0001, respectively. These correlations may serve as useful models to predict the maximally tolerated dose of an investigational anticancer agent prior to entry into clinical trials and to assist in the selection of clinically relevant in vitro CXTs for new-agent screening against human tumors.

Experimental antitumor activity of the amsacrine analogue CI-921.

CI-921 is a di-substituted analogue of amsacrine currently in phase 1 clinical trial. CI-921 was developed to clinical trial largely on the basis of a series of studies at five cancer research laboratories that demonstrated its improved spectrum and degree of activity relative to those of amsacrine against murine tumor models. The tumor models studied included lung, colon, and mammary carcinomas and encompassed a wide range of biologic properties and chemosensitivities. CI-921 had significant activity against 16 of 19 (84%) tumor models examined. The activity of CI-921 was superior to that of amsacrine in 10 of 14 tumor systems that were sensitive to at least one of the agents and for which comparable data existed. In the remaining four systems, CI-921 and amsacrine were equivalent in activity. CI-921 was found to be roughly equipotent with amsacrine on a milligram-per-kilogram (body wt) basis and was found to have significantly higher activity when given orally.

Growth support of small B16 melanoma implants with nitrosourea-sterilized fractions of the same tumor.

B16 melanoma cells sterilized in vitro with 1,3-bis(2-chloroethyl)-1-nitrosourea or in vivo with trans-1-(2-chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea, have been used to enhance the percentage of tumor takes with small s.c. implants of viable cells and to reduce the latent period between tumor implantation and palpability. The admixture of trans-1-(2-chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea-inactivated cells with viable cell implants reduced the number of cells required to produce tumors in 50% of the animals by approximately 3 log10 units and markedly reduced the time of tumor appearance from implants of up to 10(6) cells. Similar results were obtained with 1,3-bis(2-chloroethyl)-1-nitrosourea-sterilized cells. The growth-supporting effect obtained with the nitrosourea-inactivated cells appeared to be as pronounced as that previously reported, for this tumor system, with radiation-inactivated cells.

Response of drug-sensitive and -resistant L1210 leukemias to high-dose chemotherapy.

Alkylating agent-sensitive and -resistant L1210 leukemia cell lines were used to determine the tumor response to dose levels of drugs that exceeded conventional doses up to a factor of 10. Since those dose levels were lethal to the host mice, tumor response was based on the results of in vivo bioassays of spleen and/or tumor from drug-treated and control mice. When mice bearing about 10(8) drug-sensitive leukemic cells were treated with a single, conventional (approximately 10% lethal) dose of cis-diamminedichloroplatinum, L-phenylalanine mustard (melphalan), or 1,3-bis(2-chloroethyl)-1-nitrosourea, 10(1) to 10(4) tumor cells were recovered by bioassay. Treatment at doses that were 2 to 8 times the 10% lethal dose of either of those drugs resulted in no recoverable cells and survival of all bioassay recipient mice. Mice bearing advanced L1210 leukemia resistant to cis-diamminedichloroplatinum (L1210/DDPt), 1,3-bis-(2-chloroethyl)-1-nitrosourea (L1210/BCNU), cyclophosphamide (L1210/CPA), or melphalan(L1210/L-PAM) also were treated with a 10% lethal dose and greater doses of the drug to which the tumor line was resistant. Bioassay results indicated a direct correlation between dose intensity and tumor cell kill, the response being linear. Similarly, when mice with L1210/BCNU were treated with high doses of N-(2-chloroethyl)-N''-(2,6-dioxo-3-piperidinyl)-N-nitrosourea or 1,1',1''-phosphinothioylidynetrisaziridine (thioTEPA) and when mice with L1210/DDPt were treated with cyclophosphamide, an increasing, linear cell kill resulted throughout the high-dose range. Overall, these results indicate that resistance to these alkylating agents can be overcome by dose intensification and that the tumor response is linear in relation to increasing dose level.

Preclinical antitumor activity of penclomedine in mice: cross-resistance, schedule dependence, and oral activity against tumor xenografts in brain.

Penclomedine is 3,5-dichloro-2,4-dimethoxy-6-(trichloromethyl)pyridine (NSC 338720), an alpha-picoline derivative with p.o. antitumor activity in preclinical leukemia and solid tumor models. Described here are an in vivo cross-resistance profile of penclomedine, treatment schedule dependence studies, and studies exploring the effects of p.o. drug on human tumors xenografted into mouse brain. The latter studies exploited the apparent facile distribution of penclomedine to the central nervous system. Tumor models used included murine leukemia lines selected in vivo for acquired resistance to various antitumor drugs and the human mammary and lung tumor xenografts MX-1 and H82, respectively. The therapeutic effects of p.o. penclomedine against s.c. MX-1 and H82 xenografts were shown to be independent of treatment schedule. Therapeutic activity was comparable when p.o. and parenteral treatments were compared. Lines of P388 leukemia resistant to melphalan, cyclophosphamide, and carmustine were cross-resistant to penclomedine in vivo. Leukemia lines resistant to antimetabolites, DNA binders/intercalators, and vincristine were not cross-resistant to penclomedine. Intracerebrally implanted MX-1 xenografts retained their sensitivity to p.o. penclomedine, and therapeutic activity was at least comparable to that of carmustine, a drug known for its ability to cross the blood-brain barrier. These results demonstrate attributes of penclomedine that are relatively uncommon among currently available antitumor drugs and that are of interest for the anticipated clinical development of this drug.

Cyclophosphamide-adriamycin combination chemotherapy of transplantable murine tumors.

The two-drug combination of cyclophosphamide plus adriamycin was found to be therapeutically potentiating against four different C3H mammary tumor lines, the B16 melanoma, the Ridgway osteogenic sarcoma, and the P388 leukemia. The potentiation was not schedule dependent.

Tumor induction relationships in development of transplantable cancers of the colon in mice for chemotherapy assays, with a note on carcinogen structure.

In an effort to establish an animal colon tumor model suitable for biological and chemotherapy studies, 82 colon tumors were induced and transplanted in four different inbred strains of mice. Four colon tumors survived the first transplant and are now in serial passage. All are suitable for chemotherapy trials. Two tumors are highly metastatic, and at least one of these is known to be suitable for surgery-chemotherapy adjuvant studies. The effective colon carcinogens contained a (see article) molecular similarity.

Preclinical antitumor activity and pharmacological properties of deoxyspergualin.

A new antibiotic, deoxyspergualin (DSG), demonstrated antitumor activity against L1210 leukemia in mice. The life span of mice bearing either i.p. or s.c.-implanted L1210 increased greater than 150% following i.p. administration of 25 mg/kg DSG on days 1-9. Activity obtained with i.p. bolus treatments was schedule dependent. The tumor burden in mice bearing the s.c. implanted L1210 was reduced by 4-6 log10 units at the end of treatment when DSG was administered every 3 h for 8 injections on days 1, 5, and 9. By contrast, single injections of DSG on days 1, 5, and 9 allowed the tumor burden to increase at least 100-fold during treatment and daily single injections for 9 days reduced the tumor burden by 2 log10 units. The therapeutic advantage for i.p.-implanted L1210 of maintaining plasma concentrations of DSG was indicated further by infusion studies using s.c.-implanted Alzet osmotic pumps. Tumor burden was reduced by 3.5 and 6 log10 units following s.c. bolus treatments every 3 h on day 1 and a 24 h-infusion, respectively. The optimal infusion time for an infusion rate in mice of 179 mg/kg/day appeared to be 72 h. Pharmacokinetic studies following bolus i.v. injection revealed a rapid plasma clearance of parent drug (20.8 ml/min/kg) and a beta half-life of approximately 12 min. The bolus dose kinetics was used to predict the steady state plasma concentrations resulting from s.c. infusion; good agreement was observed between predicted values and experimental results. Based on these preclinical data, DSG has been developed to clinical trial. Initial Phase I protocols involve a 120-h infusion schedule.

Antitumor activity of ethyl 5-amino-1,2-dihydro-2-methyl-3-phenyl-pyrido [3,4-b]pyrazin-7-ylcarbamate, 2-hydroxyethanesulfonate, hydrate (NSC 370147) against selected tumor systems in culture and in mice.

Ethyl 5-amino-1,2-dihydro-2-methyl-3-phenylpyrido[3,4-b]pyrazin- 7-ylcarbamate, 2-hydroxyethanesulfonate, hydrate (NSC 370147) was evaluated for antitumor activity against a spectrum of tumor systems in culture and in mice. NSC 370147 was cytotoxic to a variety of mouse and human cell lines at nanomolar concentrations. The compound exhibited good in vivo antitumor activity against several murine tumors (P388 and L1210 leukemia, colon 11/A and 36, mammary 16/C, and M5076 sarcoma). Activity was largely independent of route of administration but favored a prolonged treatment schedule. NSC 370147 was as active against murine leukemia sublines resistant to Adriamycin, amsacrine, vincristine, melphalan, cisplatin, methotrexate, and CI-920 (a topoisomerase II inhibitor) as against the corresponding parental lines. Only the 1-beta-D-arabinofuranosylcytosine-resistant P388 subline exhibited any cross-resistance to NSC 370147. NSC 370147 has a spectrum of activity similar to that of vincristine and, unlike vincristine, is active against multidrug-resistant cell lines. Therefore, NSC 370147 is a candidate for clinical trial because of its favorable activity compared to vincristine, its effectiveness against multidrug-resistant cells, and its retention of activity for p.o. administration.

Preclinical antitumor activity of an alpha-picoline derivative, penclomedine (NSC 338720), on human and murine tumors.

Penclomedine, a synthetic alpha-picoline derivative, was identified as a potential antitumor agent in the P388 leukemia prescreen of the National Cancer Institute. Upon further evaluation in the National Cancer Institute in vivo tumor panel, the compound demonstrated good activity against two breast tumors. A single i.p. dose or five daily doses caused partial regressions of advanced-stage s.c. implanted mouse CD8F1 mammary adenocarcinomas. Also, penclomedine administered i.p. on Days 1,5, and 9 caused regression of the human MX-1 mammary carcinoma implanted under the renal capsule of athymic mice. In contrast, penclomedine demonstrated only marginal to moderate activity against the i.p. implanted L1210 leukemia and M5076 sarcoma and was inactive in three additional non-breast tumor models (i.p. B16 melanoma, i.v. Lewis lung carcinoma, and s.c. colon adenocarcinoma 38). Penclomedine administered p.o. and i.p. was equally effective against the subrenal capsule MX-1. Doses given p.o. every fourth day caused complete regression of 39 of 40 advanced-stage s.c. implanted MX-1 tumors but were much less effective against human H82 small cell lung carcinomas (13 of 80 complete regressions). Penclomedine p.o. also inhibited growth of the human MCF-7 and mouse 16/C breast adenocarcinomas. Further studies to support the development of penclomedine to clinical trial are in progress.

Antitumor activity of 2-chloroethyl (methylsulfonyl)methanesulfonate (clomesone, NSC 33847) against selected tumor systems in mice.

Clomesone was evaluated for antitumor activity against a spectrum of animal tumor models. Clomesone exhibited significant antitumor activity against the murine L1210 leukemia implanted i.p., s.c., and intracerebrally (i.c.). Activity against s.c.-implanted tumor was largely independent of schedule and route of administration. Therapeutically optimal single-dose treatment (for tumored mice) was less toxic to nontumored mice than therapeutically optimal prolonged treatment. Clomesone also exhibited activity against other murine tumors (P388 leukemia, B16 melanoma, Lewis lung carcinoma, and M5076 sarcoma). It was active against P388 leukemia sublines resistant to cyclophosphamide, L-phenylalanine mustard, and cis-diamminedichloroplatinum(II). No activity was observed against a P388 subline resistant to N,N'-bis(2-chloroethyl)-N-nitrosourea or against Ridgway osteogenic sarcoma, a nitrosourea-resistant murine solid tumor. Clomesone is generally as effective as the chloroethylnitrosoureas against experimental tumor models. Since clomesone does not have the hydroxyethylating and carbamoylating activities of the chloroethylnitrosoureas (which do not appear to contribute to antitumor activity), it would likely be a more toxicologically selective compound. It may prove to be less carcinogenic than the chloroethylnitrosoureas, and it may contribute less target organ toxicity and less interference with the actions of other drugs when used in combinations.

Practical spontaneous metastasis model for in vivo therapeutic studies using a human melanoma.

In vivo studies aimed at therapy of spontaneous human tumor metastases have been hampered by the lack of practical experimental models. The LOX amelanotic melanoma model described here represents a transplantation model which rapidly and reproducibly results in spontaneous pulmonary metastasis following s.c. inoculation into athymic mice. Pulmonary lesions can be detected using a simple bioassay procedure which is useful for estimation of metastatic cell killing. Using this model we demonstrate that systemic therapy with cyclophosphamide or dacarbazine can produce metastatic cell killing consistent with complete eradication of established pulmonary metastases. This model may also prove useful for future experimental therapeutic studies aimed at prevention of metastases by manipulating tumor staging interval and treatment schedule.

Toxicity and anticancer activity of a new triazine antifolate (NSC 127755).

A new triazine folate antagonist, 3-chloro-4-((4-(2-chloro-4-[4,6-diamino-2,2-dimethyl-s-triazin-1(2H)-yl]phenyl)butyl)) benzenesulfonyl fluoride compounded with ethanesulfonic acid (1:1) (NSC 127755), was highly active against four transplantable colon adenocarcinomas (36, 38, 10/A, 12/A) and the Dunning murine ovarian tumor M5076. Treatment schedule studies indicated that a prolonged time of exposure provided optimum antitumor activity for the compound. The combination of NSC 127755 plus 4-amino-1-[5-O-(1-oxohexadecyl)-beta-D-arabinofuranosyl]-2(1H)-pyrimidinone (palmO-ara-C, NSC 135962) was found to have therapeutic synergism against grossly evident colon adenocarcinoma 36.

Induction and chemotherapeutic response of two transplantable ductal adenocarcinomas of the pancreas in C57BL/6 mice.

Following implant of cotton thread-carrying 3-methyl-cholanthrene into the pancreas tissue of 90 C57BL/6 and 60 BALB/c mice, 13 developed ductal adenocarcinomas. Two of these tumors, both of C57BL/6 origin (Panc 02 and 03), were established in serial s.c. transplant. Panc 02 was treated with 37 different anticancer drugs representing all of the chemical and functional classes of clinically useful anticancer agents including alkylating agents, antimetabolites, agents that bind to or cause scission of DNA, and others that inhibit mitosis or inhibit protein synthesis. When drug treatment was started within 3 to 4 days after tumor implant, Panc 02 showed only limited response to treatment with two nitrosoureas, [N'-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-N-(2-chloroethyl)-N- nitrosourea, monohydrochloride and N-(2-chloroethyl)-N'-(2,6-dioxo-3-piperdinyl)-N-nitrosourea)], and N-phosphonacetyl-L-aspartate. Drug response of Panc 03 was determined only with Adriamycin, 5-fluorouracil, cyclophosphamide, cis-(SP-4-2)-diamminedichloroplatinum, or N,N'-bis(2-chloroethyl)-N-nitrosourea. When drug treatment was started 3 days after tumor implant, high cure rates were obtained with Adriamycin treatment, and limited therapeutic responses were seen to treatment with cis-diamminedichloroplatinum or cyclophosphamide. A comparison of the biological characteristics and drug responsiveness of Panc 02 and Panc 03 with those of a number of other transplantable tumors of mice is reported.

Evaluation of trans-tetrachloro-1,2-diaminocyclohexane platinum (IV) in murine leukemia L1210 resistant and sensitive to cis-diamminedichloroplatinum (II).

trans-Tetrachloro-1,2-diaminocyclohexane platinum (IV) (tetraplatin) was therapeutically effective in mice bearing leukemia L1210 resistant (L1210/DDPt) or sensitive (L1210/0) to cis-diamminedichloroplatinum (II) (cisplatin). Furthermore, the sensitivity of cultured L1210/DDPt and L1210/0 cell populations to tetraplatin, cisplatin, and dichloro-trans-dihydroxyisopropylamine platinum (IV) (CHIP) was a function of the concentrations used for each compound. The relative degree of sensitivity between cultured L1210/DDPt and L1210/0 cells for each compound on the basis of the LC99 (the concentration of each compound required to reduce the number of viable cells by 99% in each cell line) was 3-fold for cisplatin, 2-fold for tetraplatin, and 3-fold for CHIP; thus the cultured L1210/0 cells exhibited a greater degree of sensitivity than the L1210/DDPt cells to the platinum compounds. The data indicate that if reduction of platinum IV compounds to platinum II compounds or metabolites is required for antitumor activity, then the cultured L1210 cells are capable of this bioreduction independently of any host factors.

Cell cycle effects of trimetrexate (CI-898).

The cell cycle phase specificity of trimetrexate (CI-898) was examined. CHO cells synchronized by mitotic selection were exposed to 50 microM trimetrexate for 2 h at various time points after release from Colcemid block. Only S phase cells were sensitive to trimetrexate when survival was measured by a cloning assay. Comparison of plateau phase and log phase cultures indicated that plateau phase CHO cells were relatively insensitive to 5 microM trimetrexate. Exponentially growing L1210 cells were continuously exposed to either 30 nM or 3 nM trimetrexate and analyzed by DNA flow cytometry. Incubation with 30 nM trimetrexate produced cell cycle arrest in late G1 or early S phase, while exposure to 3 nM trimetrexate caused only a delay in progression through S phase. In an in vivo schedule dependence study with mice bearing approximately 3 X 10(6) P388 leukemia cells, trimetrexate was most effective with frequent administration. Mice treated on the optimal schedule, every 3 h X 8 on days 1, 5, and 9 after tumor implant, had life-span increases in excess of 100%.

Schedule dependence, activity against natural metastases, and cross-resistance of pyrazine diazohydroxide (sodium salt, NSC 361456) in preclinical models in vivo.

Pyrazine diazohydroxide (sodium salt, NSC 361456; PZDH) is a new antitumor drug with relatively broad activity in initial evaluations against murine leukemias, solid tumors, and two human tumor xenografts in vivo. The present studies were designed to address questions about PZDH activity on different treatment schedules, its activity against metastases, and the extent of its cross-resistance with established drugs. Human LOX amelanotic melanoma xenografts in athymic mice were used to explore schedule dependence and activity against natural metastases, and a series of drug-resistant murine leukemias provided an in vivo cross-resistance profile. Single-dose treatment and prolonged treatment provided equivalent therapeutic responses to PZDH by both the i.p. and i.v. routes in the i.p. LOX model. A s.c. LOX model resulting in spontaneous pulmonary metastases was adapted for bioassay and quantitation of the numbers of LOX cells killed by PZDH among both primary and metastatic cell populations. It was demonstrated that PZDH afforded about 2-log10 orders of magnitude greater cell kill among pulmonary metastases than against primary s.c. LOX tumors in the same mouse. Murine leukemias resistant to doxorubicin (ADR), vincristine (VCR), cisplatin (DDPt), methotrexate (MTX), N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU), and cyclophosphamide (CPA) were not cross-resistant to PZDH. However, both P388 and L1210 leukemia sublines resistant to melphalan (L-PAM) were cross-resistant to PZDH, suggesting that patients previously treated with L-PAM might have less likelihood of response to PZDH than those who had had no opportunity to develop L-PAM resistance. Although these observations should not be applied to clinical studies without due caution, they support clinical evaluation of PZDH as well as continued investigation of its molecular pharmacology.

Antitumor drug cross-resistance in vivo in a murine P388 leukemia resistant to ethyl 5-amino-1,2-dihydro-2-methyl-3-phenylpyrido[3,4-b]pyrazin-7 - ylcarbamate 2-hydroxyethanesulfonate hydrate (NSC 370,147) 370147.

Ethyl 5-amino-1,2-dihydro-2-methyl-3-phenylpyrido[3,4-b]pyrazin-7- ylcarbamate 2-hydroxyethane-sulfonate hydrate (NSC 370147) is a potent mitotic inhibitor, which has provided the basis for a candidate for clinical trial. As observed with clinically useful drugs, the development of clinical resistance to NSC 370147 will probably be encountered. Information concerning resistance to NSC 370147 should aid in the design of strategies for the optimal clinical use of the drug. A P388 leukemia resistant to NSC 370147 (P388/NSC 370147) was isolated and its in vivo cross-resistance profile was determined. The P388/NSC 370147 line was cross-resistant to vincristine but was not cross-resistant to doxorubicin, etoposide, cisplatin, melphalan, methotrexate, or 5-fluorouracil. This information plus other in vivo cross-resistance data [Waud et al. (1990) Cancer Res 50: 3239] suggests that NSC 370147 may be useful in non-cross-resistant combinations with doxorubicin, melphalan, cisplatin, or methotrexate. The lack of cross-resistance of P388/NSC 370147 to doxorubicin and etoposide shows that resistance to NSC 370147 does not involve multidrug resistance and suggests that the mdr1 gene is not involved in resistance to NSC 370147.

Effect of homoharringtonine on the viability of murine leukemia P388 cells resistant to either adriamycin, vincristine, or 1-beta-D-arabinofuranosylcytosine.

Cultured murine leukemia P388 cell populations were derived from P388 cells resistant to vincristine (P388/VCR), adriamycin (P388/ADR), and 1-beta-D-arabinofuranosylcytosine (P388/ARA-C) that were developed in vivo and to the parental drug-sensitive cells (P388/O) that were passaged in vivo. The doubling times of the cultured cell populations (mean +/- SD) between cell densities of 5 x 10(4) and 1 x 10(6) cells/ml were 14.2 +/- 2 h (P388/O), 16.5 +/- 1.9 h (P388/VCR), 16.9 +/- 1.2 h (P388/ADR), and 15.0 +/- 1.4 h (P388/ARA-C). Exponentially proliferating cultured cell populations were exposed to selected homoharringtonine (HHT) concentrations for 24 h and the surviving cell fractions were determined by colony formation in semisolid medium. The results, based on differential sensitivity of the cell populations to HHT, indicated that cultured P388/VCR cells were cross-resistant to 0.018-1.8 micrograms/ml HHT, P388/ADR cells were cross-resistant to 0.058-1.8 micrograms/ml HHT, and P388/ARA-C cells were collaterally sensitive to 0.09-0.36 micrograms/ml HHT. The results with the cultured P388/VCR, P388/ADR, P388/ARA-C, and P388/O cell populations were confirmed in animal experiments. CD2F1 mice bearing intraperitoneal (i.p.) implants of 1 x 10(6) P388/VCR, P388/ADR, P388/ARA-C, or P388/O leukemia cells were given HHT i.p. qd on days 1-9 postimplantation. Optimal treatment (less than or equal to LD10) produced in vivo cell kills of 2 to 3 log10 units in P388/O and about 7 log10 units in P388/ARA-C, whereas P388/VCR and P388/ADR cells actually increased by 1-2 log10 units during treatment. The results of this study indicate that cross-resistance (P388/VCR and P388/ADR) or collateral sensitivity to HHT (P388/ARA-C) is a function of the cellular properties of the target tumor cell populations that is independent of host factors.

Antitumor drug cross-resistance in vivo in a cisplatin-resistant murine P388 leukemia.

Since 1978, over 50 clinically useful antitumor drugs or new candidate antitumor agents have been evaluated in vivo against cisplatin-resistant P388 leukemia (P388/DDPt) in our laboratories. Analysis of this data base has yielded insights into the cross-resistance, collateral sensitivity, and mechanisms of resistance of P388/DDPt. P388/DDPt was cross-resistant or marginally cross-resistant to eight agents [carmethizole.HCl, rhizoxin, dibromodulcitol, spirohydantoin mustard, hepsulfam, arabinosyl-5-azacytosine (ara-AC), tiazofurin, and deoxyspergualin]. Of these eight agents, the latter six have entered various phases of clinical trials. For these trials, it may be important to exclude or to monitor with extra care patients who have previously been treated with cisplatin. P388/DDPt was collaterally sensitive to six agents [fludarabine phosphate (2-F-ara-AMP), amsacrine (AMSA), mitoxantrone, etoposide (VP-16), batracylin, and flavone acetic acid] and, possibly, to two others (merbarone and echinomycin). These observations of collateral sensitivity suggest that a combination of cisplatin plus any one of these drugs might exhibit therapeutic synergism. Therapeutic synergism has been observed in animal models for combinations of cisplatin plus VP-16, AMSA, or mitoxantrone. The observation of collateral sensitivity for P388/DDPt to four agents (AMSA, mitoxantrone, merbarone, and VP-16) that have been reported to interact with DNA topoisomerase II suggests the possible involvement of the latter in cisplatin resistance. Both the increased sensitivity of P388/DDPt to these agents and a portion of its resistance to cisplatin could be the result of an increase in DNA topoisomerase II activity.