Radioimmunoscintigraphy and radioimmunotherapy of renal cell carcinoma xenografts

From a panel of monoclonal antibodies (MAb) reactive with human renal cell carcinoma generated by this laboratory, three (designated A6H, C5H, and D5D) were selected for in vivo studies with a nude mouse xenograft model. These studies included 131I- and 111In-labeled MAb radioimmunoscintigraphy and 131I-labeled MAb radioimmunotherapy. In the imaging studies, these radiolabeled MAb allowed visualization of subcutaneous xenografts larger than 40 mg and subrenal capsule xenografts smaller than 20 mg. Comparisons of tumor to non-tumor tissue radiolabel distribution yielded unusually high ratios and depended on the MAb-xenograft combination. The 111In-radiolabeled A6H showed increased accumulation in the liver compared with 131I-A6H, but this still did not necessitate background subtraction for good visualization of small, subrenal capsule renal cell carcinoma xenografts. Radioimmunotherapy studies with 131I-A6H in BALB/c nu/nu mice bearing established renal cell carcinoma xenografts showed a prolonged (greater than 90 days) regression in tumor burden and possible "cures," whereas three sets of control mice showed progressive and rapid increases in tumor size. These studies indicated that MAb, which show good tissue biodistribution and high imaging sensitivity, could also be capable of delivering effective radiotherapy to the tumor when "human equivalent" radiolabeled-MAb doses are used.     

Transient perfusion in human melanoma xenografts.

Studies of transplantable rodent tumours have suggested that malignant tissue might experience transient perfusion at the microvascular level. The purpose of the work reported here was to investigate whether transient perfusion can be demonstrated in xenografted human tumours. Tumours of four melanoma lines (A-07, D-12, R-18, U-25), grown orthotopically in Balb/c nu/nu mice, were included in the study. Transient perfusion was studied by using the double-fluorescent staining technique. Hoechst 33342 and DiOC7(3) were either administered simultaneously or Hoechst 33342 was administered 20 min before DiOC7(3). Detection of transient perfusion by this method requires that vessels are non-functional for at least 5 min owing to the distribution half-lives of the dyes in the blood. Usable combinations of dye concentrations were found by varying the concentrations of Hoechst 33342 and DiOC7(3) systematically. The level of perfusion mismatch following simultaneous administration of the dyes ranged from approximately 1.5% for U-25 tumours to approximately 3.0% for R-18 tumours at these combinations. Moreover, the fraction of vessels stained only with Hoechst 33342 and the fraction of vessels stained only with DiOC7(3) were not significantly different whether the dyes were administered simultaneously or sequentially. Transient perfusion could not be demonstrated in any of the tumour lines. Thus, the fraction of vessels stained only with Hoechst 33342 and the fraction of vessels stained only with DiOC7(3) were not significantly higher after sequential than after simultaneous administration of the dyes. Moreover, the vessels stained only with Hoechst 33342 and the vessels stained only with DiOC7(3) were randomly distributed within the tumours whether the dyes were administered simultaneously or sequentially. Consequently, acute hypoxia caused by transient perfusion is probably a less pronounced phenomenon in malignant tissue than previous studies of rodent tumours have suggested.     

186Re radioimmunotherapy of small cell lung carcinoma xenografts in nude mice.

A 186Re-labeled monoclonal antibody (MAb), NR-LU-10, was used for the radioimmunotherapy of a subcutaneous human small cell lung carcinoma xenograft, SHT-1, in nude mice. Biodistribution with specific and irrelevant labeled MAb demonstrated peak tumor uptake of 8% and 3% of the injected dose/g at 2 days, respectively. Dosimetry analysis predicted tumor:whole-body radiation-absorbed dose ratios of 2.43:1 for NR-LU-10 and 0.62:1 for irrelevant MAb. Single-dose toxicity screening estimated a 50% lethal dose within 30 days of 600 microCi (880 cGy of whole-body radiation). As anticipated, a multiple-dose regimen of 490 microCi in four doses over 10 days (720 cGy of whole-body radiation, eight of eight surviving greater than 30 days) was less toxic than a single bolus dose of 430 microCi (644 cGy of whole-body radiation), six of eight surviving greater than 30 days). A multidose radioimmunotherapy regimen was initiated in nude mice bearing 66-mm3 tumors (total dose, 500 to 600 microCi). Complete remissions (greater than 140 days) were achieved in three of 16 mice, and the remainder showed a mean tumor growth delay of 53 days. Matched doses with irrelevant MAb produced one remission, one treatment-related death, and a mean growth delay of only 20 days in six of eight mice. Thus, in this nonoptimal radioimmunotherapy model, significant antitumor responses were observed using a mildly toxic multiple dosing regimen.     

Adoptive cellular immunotherapy of human ovarian carcinoma xenografts in nude mice

Adoptive cell therapy with various purified populations of human lymphoid and monocytoid effector cells which have been in vitro activated with recombinant interleukin 2 and gamma-interferon was performed in an in vivo nude mouse model of human ovarian cancer. Administration i.p. of human interleukin 2-activated populations of large granular lymphocytes resulted in a significant extension of mean survival time (30 to 60 days) in this ovarian carcinoma model. In addition, T-cells activated with interleukin 2, in a similar fashion to large granular lymphocytes, also significantly prolonged survival of animals with ovarian carcinoma. In contrast, monocytes, with or without gamma-interferon activation, did not improve survival of tumor-bearing mice. In vitro results using direct isotopic release assays to measure efficacy of effectors against the ovarian cancer cells before and after activation, especially the activated natural killer cells, paralleled the effects on survival in the nude mouse model. However, the results with T-cells were somewhat inconsistent in vitro regarding their in vivo efficacy. We assume this is due to a delayed generation of activated killing in T-cells that is generated in vivo. These experimental results in this model system of human ovarian cancer indicate that transfer of autologous activated cells may have a role in the treatment of ovarian cancer patients.     

Single-Dose versus fractionated radioimmunotherapy of human colon carcinoma xenografts using 131I-labeled multivalent CC49 single-chain fvs.

The prospects of radiolabeled antibodies in cancer detection and therapy remain promising. However, efforts to achieve cures, especially of solid tumors, with the systemic administration of radiolabeled monoclonal antibodies (MAbs) have met with limited success. Using genetic engineering techniques, MAbs have been tailored to improve the therapeutic index (tumor:normal tissue ratio) in clinical radioimmunotherapy. In the present study, we investigated the potential of tetravalent ([sc(Fv)2]2) and divalent [sc(Fv)2] single chain Fvs of MAb CC49 for therapy in athymic mice bearing s.c. LS-174T human colon carcinoma xenografts. Mice received 1,000 microCi of 131I-labeled [sc(Fv)2]2 or 131I-labeled sc(Fv)2, either as a single injection on day 6 or as four injections (250 microCi each) on days 6, 7, 8, and 9; the day of tumor implantation was taken as day 0. The median survival for the control group was 26 days. Comparisons of single and fractionated therapeutic regimens showed median survival as 32 (P < 0.001) and 53 days (P < 0.0001), respectively for [sc(Fv)2]2 and 26 (P > 0.5) and 38 days (P < 0.0001), respectively for sc(Fv)2 when compared with the control groups. The time for the quadrupling of tumor volume for single and fractionated therapeutic treatments were: 9.0 +/- 0.8 and 21.1 +/- 2.9 days respectively for sc(Fv)2; 16.6 +/- 1.9 and 32.9 +/- 2.7 days respectively for [sc(Fv)2]2; and 8.3 +/- 0.7 and 8.4 +/- 0.6 days respectively for the control group. No 131I-labeled systemic toxicity was observed in any treatment groups. The results show that radioimmunotherapy delivery for sc(Fv)2 and [sc(Fv)2]2 in a fractionated schedule clearly presented a therapeutic advantage over single administration. The treatment group receiving tetravalent scFv showed a statistically significant prolonged survival with both single and fractionated administrations suggesting a promising prospect of this reagent for cancer therapy and diagnosis in MAb-based radiopharmaceuticals.     

Hypoxia and reoxygenation in human melanoma xenografts

Tumor hypoxia and reoxygenation pattern following single dose (10.0 Gy) and fractionated (7 fractions of 2.0 Gy, 1 fraction per day) irradiation were studied in five human melanoma xenograft lines using the paired survival curve method. The hypoxic fractions differed significantly among the melanoma lines; they were found to be 6 +/- 3% (E.E.), 22 +/- 8% (E.F.), 31 +/- 11% (G.E.), 45 +/- 17% (M.F.), and 15 +/- 5% (V.N.). There were no clear correlations between hypoxic fraction and tumor volume-doubling time or vascular density, suggesting that intrinsic cellular characteristics, for example, rate of oxygen consumption and ability to retain clonogenicity under hypoxic stress, also may play an important role for the magnitude of the hypoxic fractions in the melanomas. Reoxygenation was rapid and extensive in all melanoma lines; 12-24 hr after the single dose irradiation or the last fraction of the fractionated irradiation, the hypoxic fractions were similar to those in untreated tumors and stayed at that level up to at least 10 days after irradiation. The hypoxic fractions 1-10 days after irradiation tended to be higher after fractionated than after single dose irradiation, but the differences were not statistically significant. There was a positive correlation between the hypoxic fractions in untreated tumors and the hypoxic fractions after irradiation and reoxygenation, suggesting that it may be possible to predict radiation resistance caused by hypoxia from the hypoxic fractions in tumors before start of radiation therapy. However, hypoxia is probably not a major cause of failure in the radiation therapy of malignant melanoma.     

Repopulation between radiation fractions in human melanoma xenografts.

Rate of tumor repopulation between radiation fractions was studied using two human melanoma xenograft lines (E.F. and V.N.). Tumors were given five radiation fractions under hypoxic conditions in vivo and clonogenic cells survival was measured in vitro after the last radiation fraction. Dose-response curves were established for interfraction times of 12, 24, 36, and 48 hr by varying dose per fraction from 4.2 to 11.2 Gy. Assuming an oxygen enhancement ratio of 2.8, these doses corresponded to doses of 1.5 to 4.0 Gy under aerobic conditions, that is, clinically relevant doses per fraction were used. The dose-response curves were nearly parallel and were shifted to the right with increasing interfraction time, demonstrating significant repopulation between the radiation fractions. Iso-effect analyses showed that additional radiation doses of 2.0 +/- 0.6 Gy/day (E.F.) and 2.2 +/- 0.6 Gy/day (V.N.), corresponding to doses of 0.7 +/- 0.2 Gy/day (E.F.) and 0.8 +/- 0.2 Gy/day under aerobic conditions, would be required to compensate for the repopulation. These doses were equivalent to the surviving clonogenic cells showing doubling times of 40-50 hr (E.F.) and 30-40 hr (V.N.) during the treatment period. The radioresponsiveness of the two melanoma xenograft lines was also measured. Tumors in air-breathing mice were given from 5 to 15 daily fractions of 2.0 Gy and cell survival curves were established in vitro. Theoretical survival curves, calculated from SF2 in vitro and rate of repopulation during fractionated irradiation in vivo, agreed fairly well with the measured survival curves. This suggested that the radioresponsiveness of the melanoma xenograft lines was governed by two main parameters: (a) cellular radiation sensitivity and repair capacity and (b) rate of repopulation between radiation fractions.     

Radiobiological comparison of external beam irradiation and radioimmunotherapy in renal cell carcinoma xenografts

Growth delay was measured in TK-82 renal cell carcinoma (RCC) xenografts implanted in nude mice receiving single fraction external beam irradiation (SF-XRT), multifraction external beam irradiation (MF-XRT), or radioimmunotherapy (RIT). Thermoluminescent dosimeter(s) (TLD) and autoradiography were used to ascertain the average absorbed dose delivered and the degree of heterogeneous uptake of radiolabeled antibody for the RIT irradiations. For intravenous administered activities of 100, 200, 400, and 600 microCi of I-131 labeled A6H antibody, volume doubling times (VDT) and TLD absorbed dose measurements for each administered activity were 7 days (341 cGy), 38 days (383 cGy), 85 days (886 cGy) and no regrowth (1034 cGy), respectively. For SF-XRT irradiations of 500, 1000, and 1500 cGy, VDT times were 11, 62, and 103 days, respectively. MF-XRT of 4 X 250 cGy over a 2-week period yielded a VDT of 25 days. Marked peripheral activity deposition was noted on most autoradiographs from multiple tumor samples. These data suggest that an equivalent to superior tumor growth delay is obtained for absorbed doses delivered by exponentially decaying low dose rate radioimmunotherapy RIT compared to similar doses of acute dose rate XRT as quantitated by the TLD method.     

Successful local regional therapy with topotecan of intraperitoneally growing human ovarian carcinoma xenografts.

The therapeutic effects of intraperitoneal topotecan, a water-soluble camptothecin analogue, were investigated in two models of human ovarian carcinoma xenografted intraperitoneally into nude mice: the IGROV-1 tumour, which originated from an untreated patient, and the A2780 tumour, selected for resistance in vitro to cisplatin (A2780DDP). In IGROV-1 tumour-bearing mice, the optimal dose (10 mg kg-1) of topotecan, given intraperitioneally every 4 days for four occasions markedly increased survival time over control mice (300 T/C%) and cured 4/9 mice, and such effects were not achieved by any of the clinically available drugs tested, i.e. cisplatin carboplatin and doxorubicin delivered intraperitonally according to their optimal doses and schedules. In the treatment of A2780DDP tumour-bearing mice, topotecan was very effective since, at dose levels of 6.6 and 10 mg kg-1 every 4 days for four occasions, 15/18 mice survived more than 100 days, and most of them (12/15) were found to be tumour free. The high responsiveness of this tumour to topotecan might be related to the elevated expression of the target enzyme topoisomerase I. From these results, intraperitoneal treatment with topotecan appears to be a promising approach in the therapy of refractory ovarian cancer confined to the peritoneal cavity.     

Successful radioimmunotherapy for lung metastasis of human colonic cancer in nude mice.

The potential for radioimmunotherapy as an adjuvant treatment for early disseminated colonic cancer was investigated in an experimental lung metastasis model. Nude mice receiving intravenous injection with a suspension of human colonic cancer cells (GW-39) developed multiple (10-100) tumor nodules throughout the lungs, and more than 50% of the animals died of extensive tumor involvement within 5-10 weeks. Groups of eight or nine animals bearing 7-day-old tumor transplants were treated with a single intravenous injection of radioiodinated agents: either 0.15 or 0.30 mCi of whole IgG of the NP-4 murine monoclonal antibody (MAb) against carcinoembryonic antigen (CEA) or 0.15 or 0.30 mCi of whole IgG of Immu-31, an anti-alpha-fetoprotein (anti-AFP) MAb. Treatment of animals with 0.15 or 0.30 mCi of 131I-labeled NP-4 IgG 7 days after injection of tumor cells resulted in survival for 23 weeks after tumor implantation in four of eight and seven of nine animals, respectively. Microscopic examination revealed that over 90% of the lung tumor colonies had no evidence of surviving cells. Animals treated with 0.30 mCi of anti-AFP, an irrelevant MAb, survived 4 weeks longer than controls. Toxicity was evident in four of the 17 animals given 0.30 mCi of NP-4 IgG (specific) or anti-AFP IgG (irrelevant) MAb. These animals died within 1-3 weeks after radioantibody injection, suggesting that death was related to the radiation dose. None of the animals given 0.15 mCi of 131I-MAb died within this period.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth and metastasis of human melanoma xenografts in the hamster host.

Cells from three human melanoma cell culture lines (UCLASO-M-12, UCLASO-M-7, and WEG-1) were transplanted into the cheek pouches of immunosuppressed hamsters. Tumor nodules were found in the cheek pouches of hamsters receiving any one of these lines, and by 90--100 days after transplantation, nearly all hamsters had grossly visible lung metastases. Metastases were noted only in hamsters receiving continuous immunosuppression during the transplant period.     

Spatial distribution of tumor-specific monoclonal antibodies in human melanoma xenografts.

The time-dependent (1-72-h) spatial distribution of three biotinylated anti-melanoma monoclonal antibodies (MAbs), a control MAb, and several macromolecular tracers was studied in two small (4-12-mg), well-characterized human melanoma xenografts (SK-MEL-2, M21) growing in the s.c. space of athymic nude mice. The specific MAbs (436, IND1, and 9.2.27) recognize two different melanoma cell surface antigens (Mr 125,000 glycoprotein melanoma-associated antigen and high molecular weight melanoma-associated antigen) and have equilibrium association constants differing by two orders of magnitude (10(8)-10(10) M-1). SK-MEL-2 tumors were poorly vascularized and were composed of one or several collections of tumor cells with few intratumor blood vessels. In contrast, M21 tumors induced a strong angiogenic response and were organized into multiple small tumor cell nests separated from each other by fine blood vessels. Neither tumor developed extensive connective tissue stroma. In both tumors, hyperpermeable blood vessels were concentrated at the tumor-host interface but some intratumor vessels in M21 tumors were also leaky. Macromolecular tracers extravasated extensively from leaky vessels into tumor stroma but penetrated poorly into tumor parenchyma. All three tumor-specific MAbs stained tumor cell surfaces in a time-dependent fashion such that one-half or more of all tumor cells were stained by 24-48 h. Tumor cell staining was favored by increased density of tumor cell antigens but, at the doses studied, was little affected by differences in affinity among tumor-specific antibodies. The distribution of MAb staining was nonuniform in two respects: (a) peripherally situated tumor cells were more likely to be stained than centrally placed cells, and only in the smallest tumors did MAb reach centrally placed tumor cells; and (b) staining was nonuniform in different parts of the same tumor. The inhomogeneity of tumor cell staining by tumor-specific MAb was attributable to several factors, including: tumor blood vessel number, distribution, perfusion and permeability; distribution of tumor connective tissue stroma; small volume of the parenchymal interstitial space and relatively impaired diffusion of macromolecules in that space (low effective diffusivity of MAb); and interactions between specific MAbs and tumor cells. Of these factors, those associated with the parenchymal compartment apparently were rate limiting, and strategies that enhance parenchymal penetration are likely to improve solid tumor therapy with MAbs.     

Serial cytogenetic studies of human colonic tumour xenografts

Chromosome studies have been made of 2 human colonic tumour lines maintained as xenografts in immune-deprived mice. In both tumours human karyotypes were retained, although progressive changes occurred during serial passage. In one tumour, independent gain of a chromosome 19 was found in the stemline and 2 sidelines. In the other tumour there was selection for a sideline containing a particular deleted marker chromosome. The advantages of chromosome analysis in a xenograft system, both for the study of human solid tumour karyotypes and for monitoring the continued presence of the human genome, are discussed.     

Sphingomyelin enhances chemotherapy efficacy and increases apoptosis in human colonic tumor xenografts.

Evidence suggests that ceramide, generated from a distinct subcellular pool of sphingomyelin (SM) by the action of sphingomyelinases, may be used by cells to propagate apoptotic signals in response to a variety of cytotoxic agents. Since most tumor cells have altered lipid metabolism, it is possible that the intracellular pool of SM used for signaling is decreased. To overcome this, we have attempted to increase the SM content of all intracellular compartments with exogenous SM and examined the impact on 5-fluorouracil (5FU) and irinotecan chemosensitivity. Our data suggest that the efficacy of these two chemotherapeutics for the treatment of HT-29, HCT15 and GW-39 human colonic tumor xenografts can be enhanced by the use of exogenous SM. Furthermore, this enhancement may be due to a reversal of the attenuation of the apoptotic signal found in cancer cells without inducing significant hematopoietic, hepatic or renal toxicity.     

beta-Hexosaminidase isozymes in human colonic carcinoma.

Isoelectric focusing of crude extracts demonstrated that human colonic carcinomas contained a higher proportion of beta-hexosaminidase B than beta-hexosaminidase A, while normal human colonic mucosa contained a higher proportion of the A form of the enzyme. Studies of the isolated isozymes showed that beta-hexosaminidase B was more stable to heat and more active at low pH than beta-hexosaminidase A. Kinetic studies revealed that the A and B forms of beta-hexosaminidase had essentially the same Km and Vmax for p-nitrophenyl-N-acetyl-beta-D-glucosaminide.     

Proton relaxation times and interstitial fluid pressure in human melanoma xenografts.

The interstitial fluid pressure (IFP) and the proton spin-lattice and spin-spin relaxation times (T1 and T2) of some experimental tumours have been shown to be related to tumour water content. These observations have led to the hypothesis that magnetic resonance imaging (MRI) might be a clinically useful non-invasive method for assessment of tumour IFP. The purpose of the work reported here was to examine the general validity of this hypothesis. R-18 human melanoma xenografts grown intradermally in Balb/c nu/nu mice were used as the tumour model system. Median T1 and T2 were determined by spin-echo MRI using a 1.5-T clinical whole-body tomograph. IFP was measured using the wick-in-needle technique. No correlation was found between tumour IFP and fractional tumour water content. Moreover, there was no correlation between median T1 or T2 and IFP, suggesting that proton T1 and T2 values determined by MRI cannot be used clinically to assess tumour IFP and thereby to predict the uptake of macromolecular therapeutic agents.     

Radiosensitization of tumor-targeted radioimmunotherapy with prolonged topotecan infusion in human breast cancer xenografts

Clinical radioimmunotherapy (RIT) of solid tumors holds great promise, but as yet has been unable to deliver tumoricidal radiation doses without unacceptable toxicity. Our experimental approach aims to potentiate the therapeutic action of radioimmunoconjugates at the tumor site and thus improve the efficacy of RIT by combination with other treatment modalities. The topoisomerase I inhibitors are a unique class of chemotherapeutic agents that interfere with DNA breakage-reunion by inhibiting the action of topoisomerase I. Preclinical studies suggest that prolonged infusion of topoisomerase I inhibitors enhances cell toxicity due to ionizing radiation. We evaluated the efficacy of combined treatment with continuous administration of topotecan and 90Y-MX-DPTA BrE3 monoclonal antibody (which recognizes an epitope of breast epithelial mucin expressed in most breast cancers) on human mammary carcinoma xenografts in nude mice. Topotecan or 90Y-BrE3 treatment alone delayed overall tumor growth rate transiently but did not affect survival. The combination of RIT with topotecan substantially reduced growth of relatively large established tumors and caused complete tumor regressions and prolonged tumor-free survival in a substantial proportion of treated animals. In vitro studies demonstrated an increase in apoptotic rate and a decrease in cell proliferation of tumor cell lines treated with this combination. We combined the radiosensitization property of topotecan and the specificity of systemic RIT to establish a novel therapy for solid tumors in an experimental tumor xenograft model.     

Effects of cyclic-nucleotide derivatives on the growth of human colonic carcinoma xenografts and on cell production in the rat colonic crypt epithelium

Previous studies have shown that various amine hormones are able to influence the growth rate of human colorectal carcinomas propagated as xenografts in immune-deprived mice, and it is now well known that the effects of many amine and other hormones are mediated by cyclic nucleotides, acting as second messengers within cells. In the present study the influence of various derivatives of cyclic adenosine monophosphate and cyclic guanosine monophosphate on the growth of two different lines of colorectal cancer growing in immune-deprived mice, and on the cell production rate in the colonic crypt epithelium of the rat, was assessed. Growth of each tumour line, as well as crypt-cell production, was suppressed by treatment wit N6O2' dibutyryl and N6 monobutyryl derivatives of cyclic adenosine monophosphate. Dibutyryl cyclic guanosine monophosphate, on the other hand, was found to promote the growth of Tumour HXK4 and to promote crypt cell production, but to have no significant effect on Tumour HXM2.     

Superior efficacy of trimelamol to hexamethylmelamine in human ovarian cancer xenografts

Summary A series of eight human ovarian cancer lines grown in nude mice were used to compare the activity of hexamethylmelamine (HMM) and N ² ,N ⁴ ,N ⁶ -trihydroxy methyl-N ² ,N ⁴ ,N ⁶ -trimethylmelamine (trimelamol). The tumor lines differed in histological subtype and growth rate. The drugs were administered i.p. at the maximum tolerated dose at alternate days. Differences in volume of treated and control tumors were endpoints of the study. The tumor lines varied widely in sensitivity to HMM and in four lines a T/C% below 25% was achieved. Trimelamol appeared to be more active than HMM and achieved a T/C below 25% in seven tumor lines. Thus far, the drug has demonstrated significant activity in a phase I trial in ovarian cancer patients. Comparative clinical studies of HMM vs trimelamol have not yet been performed.     

Acquisition of platinum drug resistance and platinum cross resistance patterns in a panel of human ovarian carcinoma xenografts.

In vivo models of acquired resistance to the platinum-based agents cisplatin (CDDP), carboplatin (CBDCA), iproplatin (CHIP) and tetraplatin have been established using a panel of six parent human ovarian carcinoma lines, two (HX/110 and PXN/87) being derived from previously untreated patients. Resistance has been generated to CDDP (three lines), CBDCA (one line), CHIP (three lines) and tetraplatin (one line) either by treatment in vivo or (for one line to CDDP) through exposure in vitro and subsequent transfer to mice. With the four tumours where resistance was generated using CDDP or CBDCA, a complete cross-resistance to the remaining platinum agents studied was observed. In contrast, in one of three lines with derived resistance to the platinum (IV) agent, CHIP, (PXN/951) a retention in sensitivity was observed with CDDP and CBDCA. Only one of the six parent tumour lines (PXN/100) was markedly sensitive to tetraplatin. Where resistance was generated to tetraplatin (PXN/100T) there was some retention of activity by CDDP. For the CDDP-resistant line established in vitro, there was a close agreement between the cross-resistance profile obtained in vitro vs that obtained in vivo. This tumour panel may be useful in the elucidation of cellular and molecular resistance mechanisms to platinum drugs operative in vivo. Moreover, as they appear to mimic the clinical observations of shared cross-resistance between CDDP, CBDCA and CHIP, they may represent valuable preclinical evaluation models for the discovery of drugs capable of conferring responses in CDDP-refractory ovarian cancer.