Development of drug resistance in a murine mammary tumour

In vitro biological evaluation of platinum(II) complexes with 1-(methoxy substituted benzyl) azetidine-3,3-dicarboxylato ligands

A number of platinum(II) complexes with ammine or 1R,2R-diaminocyclohexane as carrier ligands and 1-(methoxy-substituted benzyl) azetidine-3,3-dicarboxylate as leaving groups were synthesized and spectrally characterized. Biological evaluation in vitro showed that some of compounds showed positive antitumor activity. In particular, complex 3a, (1R,2R-diaminocyclohexane)[1-(3-methoxylbenzyl) azetidine-3,3-dicarboxylato)-O,O'] platinum(II), possessed a potent antitumor effect comparable to cisplatin and/or oxaliplatin, and very low toxicity in vivo. Preliminary antitumor mechanism of 3a has been investigated by cell apoptosis assays compared with cisplatin and oxaliplatin.

New isomeric azine-bridged dinuclear platinum(II) complexes circumvent cross-resistance to cisplatin

Four new isomeric azine-bridged complexes ([(cis-Pt(NH(3))(2)Cl)(2)(mu-pzn)]Cl(2) (1a) (pzn = pyrazine) and its corresponding nitrate salt (1b), [(cis-Pt(NH(3))(2)Cl)(2)(mu-pmn)]Cl(2) (2) (pmn = pyrimidine), and [(cis-Pt(NH(3))(2)Cl)(2)(mu-pdn)](NO(3))(2) (3) (pdn = pyridazine) have been newly synthesized as potential anticancer compounds. These complexes have been characterized by (1)H and (195)Pt NMR spectroscopy, and also the X-ray crystal structure of 1b has been determined. The reactions of 1a, 2, and 3 with guanosine-5'-monophosphate (GMP) have been monitored and kinetically investigated in D(2)O solutions at 310 K using (1)H NMR spectroscopy. Both 1a and 2 react with 2 equiv of GMP to form 1:2 complexes. The reactions involve a stepwise direct substitution of chloride ligands by GMP, with similar reaction rates for both complexes. On the other hand, the reaction of 3 with GMP results in the cleavage of one of the Pt-N(pyridazine) bonds to form an N7,O6-platinated polymer. The reaction products have been separated and have been characterized by (1)H and (195)Pt NMR spectroscopy. A cytotoxicity assay of the azine-bridged complexes (1a, 1b, 2, and 3) has been performed on human tumor cell lines and two L1210 murine leukemia cell lines (one sensitive to and one resistant to cisplatin). In general, the complexes show lower cytotoxicity than cisplatin for the human tumor cell lines except for the IGROV cell line. Their cytotoxicity for the mouse cell lines is comparable to or higher than that of cisplatin. Furthermore, these complexes appeared to largely or partly overcome the cross-resistance to cisplatin. Implications of these findings are discussed in the context of a structure-activity relationship for this class of compounds.

Hypoxia and drug resistance

Biologically and therapeutically important hypoxia occurs in many solid tumor masses. Hypoxia can be a direct cause of therapeutic resistance because some drugs and radiation require oxygen to be maximally cytotoxic. Cellular metabolism is altered under hypoxic conditions. Hypoxia can result in drug resistance indirectly if under this condition cells more effectively detoxify the drug molecules. Finally, there is evidence that hypoxia can enhance genetic instability in tumor cells thus allowing more rapid development of drug resistance cells. The current review describes the effects of hypoxia on tumor response to a variety of anti-cancer agents and also describes progress toward therapeutically useful methods of delivering oxygen to tumors in an effort to overcome therapeutic resistance due to hypoxia. Finally, the use of hypoxic cell selective cytotoxic agents as a means of addressing hypoxic 'drug resistance' is discussed.

Heterogeneity of radiosensitivity in a human glioma cell line

Sixteen clones were isolated from an early-passage human glioma cell line (IN859) and have been found to show variation in several biological characteristics including DNA content, modal chromosome number, and morphology. In addition, heterogeneity of radiosensitivity was detected: the doses that gave a surviving fraction of 0.01 varied by a factor of approximately 1.5. The most sensitive (clone 6) and the most resistant (clone 9) clones were selected for further study; their surviving fractions at 2Gy (SF2) were 0.37 and 0.64, respectively. When compared at a fixed radiation dose the sensitive clone surprisingly demonstrated greater split-dose recovery than the resistant clone; it also showed greater low dose-rate sparing.

Characteristics of five human tumor cell lines and sublines resistant to cis-diamminedichloroplatinum(II)

In order to study the mechanisms responsible for resistance to CDDP, 5 human tumor cell lines were made resistant to CDDP by repeated in vitro exposures. After cloning it was found that the cell lines developed were between 3.3-fold and 17-fold more resistant to CDDP than the parental cell lines at the IC90. These lines were also resistant to carboplatin and tetraplatin; however, resistance to tetraplatin was lower than to the other platinum complexes. Sensitivity was also assessed to Adria, MTX, 5-FU, chlorambucil, 4-HC, 4-HIF, BCNU, Thiotepa, HN2, Mito C and L-PAM, and no consistent cross-resistance was observed. As compared with the parental lines, non-protein sulfhydryl content was elevated in 3 resistant lines, and protein sulfhydryl was elevated in all 5 lines, as was glutathione-S-transferase activity. Measurements of platinum in whole cells and nuclei after exposure of the cultures to 25 microM CDDP for either 1 or 6 hr showed that nuclear levels reflected those in whole cells and that, per mg protein, platinum levels were lower in resistant cells at both time points. Formation of DNA cross-links, determined by alkaline elution, was lower in resistant cell lines than in parental cell lines, but did not correlate with the absolute cell kill observed. These results indicate that cellular resistance to CDDP often involves decreases in drug accumulation and increases in protein sulfhydryl content. Possible strategies for overcoming these mechanisms are discussed.

Development and characterisation of a cyclophosphamide resistant variant of the BNML rat model for acute myelocytic leukaemia

A cyclophosphamide resistant subline (BNML/CPR) was developed in vivo in the BN rat acute myelocytic leukaemia (BNML) model. Full resistance was achieved after in vivo exposure of leukaemic animals to cyclophosphamide with, in total, 15 intraperitoneal injections of 100 mg/kg. The CPR line was cross-resistant to ifosfamide, but less so to mafosfamide. Continuous transplantation of the BNML/CPR line without a cyclophosphamide selection pressure resulted in the emergence of a subline (BNML/CPR greater than S) whose sensitivity to cyclophosphamide was similar to that of the parent BNML/S line. Both in the BNML parent line and in the BNML/CPR greater than S line, a 2p+ marker chromosome was present, whereas a 2p+q+ marker chromosome was characteristic for the BNML/CPR line. The mechanism of cyclophosphamide resistance can now be investigated in the BNML model at the DNA, at the mRNA and at the protein level.

Inhibition of N-methyl-N'-nitro-N-nitrosoguanidine-induced methotrexate and adriamycin resistance in CHO cells by adeno-associated virus type 2

We studied the effects of helper-dependent parvovirus AAV [adeno-associated virus] type 2 on carcinogen-inducible resistance to methotrexate (MTX) and adriamycin (ADR) in Chinese hamster ovary cells. Both types of drug resistance were monitored by determination of the number of drug-resistant colonies normalized for the respective value of plating efficiency under non-selective conditions. Treatment of cells with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) drastically enhanced the frequency of resistance to MTX and ADR. By contrast, infection of cells with AAV-2 prior to treatment with MNNG markedly inhibited carcinogen-induced drug resistance. Infection by AAV alone did not exert any effect. Analysis of the dihydrofolate reductase (dhfr) gene copy numbers of individual MTX-resistant clones derived from MNNG-treated and non-treated cultures revealed similar frequencies (60-80%) and amplitudes of dhfr gene amplification (2- to 8-fold) irrespective of prior AAV treatment. Hence, carcinogen-induced enhancement of MTX-resistance could reflect an increase in the frequency of dhfr gene amplification among the survivors of MNNG treatment. On the other hand, inhibition of carcinogen-inducible drug resistance by AAV suggests an interference of the virus with cellular responses to genotoxic stress, thus leading to enhanced cell killing under altered growth conditions. Possible mechanisms responsible for the inhibitory effect of AAV and its relevance in relation to tumor chemotherapy are discussed.

Tumor resistance to alkylating agents conferred by mechanisms operative only in vivo

EMT-6 murine mammary tumors were made resistant to cis-diamminedichloroplatinum (II) (CDDP), carboplatin, cyclophosphamide (CTX), or thiotepa in vivo by treatment of tumor-bearing animals with the drug during a 6-month period. In spite of high levels of in vivo resistance, no significant resistance was observed when the cells from these tumors were exposed to the drugs in vitro. The pharmacokinetics of CDDP and CTX were altered in animals bearing the respective resistant tumors. The resistance of all tumor lines except for the EMT-6/thiotepa decreased during 3 to 6 months in vivo passage in the absence of drugs. These results indicate that very high levels of resistance to anticancer drugs can develop through mechanisms that are expressed only in vivo.

Tumor resistance to alkylating agents conferred by mechanisms operative only in vivo

EMT-6 murine mammary tumors were made resistant to cis-diamminedichloroplatinum (II) (CDDP), carboplatin, cyclophosphamide (CTX), or thiotepa in vivo by treatment of tumor-bearing animals with the drug during a 6-month period. In spite of high levels of in vivo resistance, no significant resistance was observed when the cells from these tumors were exposed to the drugs in vitro. The pharmacokinetics of CDDP and CTX were altered in animals bearing the respective resistant tumors. The resistance of all tumor lines except for the EMT-6/thiotepa decreased during 3 to 6 months in vivo passage in the absence of drugs. These results indicate that very high levels of resistance to anticancer drugs can develop through mechanisms that are expressed only in vivo.

High frequency of double drug resistance in the B16 melanoma cell line

Methotrexate (MTX) and N-(phosphonacetyl)-L-aspartate (PALA) are two agents to which cellular resistance can be conferred by gene amplification, but they do not generally show cross resistance. However, combined treatment with these two agents produced drug resistant cells in the B16 melanoma cell line at a much higher frequency than would be expected if resistance to the two agents was totally independent. An isolated doubly resistant clone, B16-F1 MP, showed a high frequency of resistance to pyrazofurin and ouabain, which are also agents to which resistance can be conferred by gene amplification. Thus MTX combined with PALA selected cells with an 'amplificator' phenotype (an increased ability to amplify parts of the genome). These B16-F1 MP cells had a decreased ability to form experimental lung metastases compared with the parent line but this difference was not found in baby hamster kidney cells with the amplificator phenotype. The mechanism underlying drug resistance may need to be considered when designing combination chemotherapy.

Characterization of a novel mouse reticular cell sarcoma M5076 subline resistant to cisplatin

A novel murine tumor resistant to cis-diamminedichloroplatinum (cisplatin, DDP) was obtained (M5/DDP) after 22 passages in which mice bearing the ovarian reticular cell sarcoma M5076 (M5) were treated with DDP. Although DDP conserved some inhibitory activity on growth of M5/DDP, it was much less effective than on M5. Treatment with DDP did not prolong the survival time of mice with M5/DDP, whereas it markedly prolonged survival of M5-bearing mice. M5 and M5/DDP tumors shared many biological and biochemical features. They were similar histologically, they metastasized reproducibly to the liver and were poorly immunogenic. Their growth rates were comparable; their DNA index, percentage of cells in S phase and intra-cellular glutathione content were also similar. In both tumors, DDP caused an accumulation of cells in S late-G2-M within 24 hr after drug treatment. However, this was efficiently reversed in M5/DDP, whereas it worsened and persisted longer in M5. Cross-resistance was observed between DDP and its analogues carboplatin and iproplatin, but tetraplatin retained marginal activity on M5/DDP tumor. Several alkylating agents tested [L-phenyalanine mustard (L-PAM); cyclophosphamide (CTX); chlorambucil (CLB); 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and dacarbazine (DTIC)] were not totally cross-resistant to DDP, but showed greater activity on M5 than on M5/DDP. Other non-alkylating anti-neoplastic drugs showed a similar degree of activity on M5 and M5/DDP. 5-Aza-2'-deoxycytidine (Aza-d-Cyd) was very effective on both tumors, etoposide (VP-16) and cytosine arabinoside (Ara-C) had no activity and Adriamycin (ADR) was weakly effective.

Development of drug resistance in a human epidermoid lung carcinoma xenograft line

The development of resistance to vincristine, actinomycin D and cisplatin has been examined in a human epidermoid lung carcinoma xenograft line (HXL 55) growing in nude mice. Treatment of HXL 55 with 1 mg kg-1 vincristine or 0.5 mg kg-1 actinomycin D once in each in vivo passage resulted in a rapid reduction in tumour responsiveness to these drugs. A partial resistance was already acquired at the 2nd transplant generation. In contrast, a gradual decrease in therapeutic response was observed with 10 mg kg-1 cisplatin. Irradiation with a local dose of 10 Gy induced no resistance. The three induced drug-resistant sublines were characterized in terms of the time course of development of resistance, the degree of induced resistance, cross-resistance, growth rate and stability of the phenotype.

Resistance and cross-resistance of the IgM immunocytoma in the LOU/M Wsl rat for cisplatin, carboplatin, and iproplatin

We investigated the antitumor activity of cis-diammine[1,1-cyclobutanedicarboxylato]platinum(II) (CBDCA, JM8) and cis-dichloro-trans-dihydroxybis(isopropylammine)platinum(IV) (CHIP, JM9) for the cis-DDP-sensitive and -resistant IgM immunocytoma in the LOU/M Wsl rat. The optimal dose for the antitumor effect of cis-diamminedichloroplatinum (cis-DDP) in this tumor model is 1 mg/kg body weight. In order to determine the dose range for antitumor activity of JM8 and JM9, tumor-bearing rats were treated i.p. (twice weekly) with 2, 4, 8, 16, or 32 mg/kg JM8 or with 2, 4, or 8 mg/kg JM9. The maximal antitumor activity of JM8 was found at a dose of 4-8 mg/kg and that of JM9, at 4 mg/kg. Doses of 16 or 32 mg/kg JM8 did not increase the antitumor activity. Recurrence of tumors was observed in JM8- and JM9-treated rats. It was demonstrated that these relapses during treatment with JM8 or JM9 involved tumor cell populations almost completely resistant against therapy with the respective drugs. The growth of cis-DDP-resistant tumors was not influenced by the analog JM9 (4 and 8 mg/kg). Only a high dose of JM8 (32 mg/kg) caused growth retardation of the cis-DDP-resistant IgM subline. The JM8-resistant tumor was resistant to treatment with cis-DDP (1 and 2 mg/kg). The JM9-resistant tumor was also resistant to this treatment (1 mg/kg); however, at a dose of 2 mg/kg cis-DDP, growth retardation of the tumor occurred. We conclude that cis-DDP, JM8, and JM9 induce resistance in the IgM immunocytoma tumor system; tumors resistant for cis-DDP were not sensitive to the treatment with JM8 or JM9. Although JM9 reacts in vitro distinctly differently with DNA than cis-DDP and JM8, no differences were found in the induction of Pt resistance. In this study tumor cells were readily made resistant, which allows us to study in more detail the induction of (cross-) resistance by cis-DDP, JM8, and JM9.

The effect of fluosol-DA and oxygenation status on the activity of cyclophosphamide in vivo

The addition of Fluosol-DA followed by carbogen breathing increased the antitumor effect of cyclophosphamide as measured by both tumor growth delay and tumor cell survival assays. Under air breathing condition, cyclophosphamide (100 mg/kg) administered i.p. five times on alternate days produced a tumor growth delay in the FSaIIC fibrosarcoma of 8.0 +/- 0.8 days. Adding Fluosol-DA (0.3 ml) to treatment with cyclophosphamide followed by carbogen breathing increased tumor growth delay to 11.4 +/- 3.6 days, which was not statistically significantly different from that obtained with the drug plus carbogen breathing without Fluosol-DA. As the dose of Fluosol-DA was increased and administered with drug treatment followed by carbogen breathing for 6 h, increasing tumor growth delays of 15.0 +/- 1.5 days, 18.1 +/- 1.7 days and 29.4 +/- 2.2 days were observed with 0.1 ml, 0.2 ml and 0.3 ml Fluosol-DA, respectively. When 0.1 ml Fluosol-DA was administered in combination with cyclophosphamide and immediately followed by 1 h of hyperbaric oxygen (3 atm), a tumor growth delay of 13.7 +/- 1.2 days was observed. With 0.2 ml Fluosol-DA under these conditions, the tumor growth delay increased to 23.2 +/- 1.6 days, and with 0.3 ml Fluosol-DA the tumor growth delay was 35.6 +/- 3.2 days. Single doses of cyclophosphamide with and without Fluosol-DA (0.3 ml) and various conditions of oxygenation were used in an FSaIIC fibrosarcoma tumor cell survival assay. The addition of Fluosol-DA to this single-dose protocol produced a five- to tenfold increase in tumor cell kill compared to air-breathing drug-treated animals. There was no significant difference in the toxic effect of any of the treatment conditions on bone marrow.

Development of MeCCNU-resistance in clonally derived lines of Lewis lung carcinoma

The development of resistance to MeCCNU in four clonal lines of Lewis lung carcinoma derived from lung colonies has been examined. Treatment of the clones with 15 mg/kg MeCCNU once in each in vivo passage resulted in a rapid reduction in tumour responsiveness to this drug. This shows that MeCCNU resistance can arise at rates which may be applicable in spontaneous tumours and their metastases. Karyotypic heterogeneity was evident very early after isolation of the clones and this has potentially important implications for the experimental use of clones and for the features of metastatic disease. Karyotype analysis of one of the clones at each passage suggested that development of resistance was associated with the selection of a single sub-population of tumour cells but the kinetics of resistance development cannot be totally explained in terms of this simple selection.

The in-vitro chemosensitivity of three cell lines derived from the VM/DK spontaneous murine astrocytoma

Three cell lines, VM/Dk P497 P540 and P560 derived from the VM spontaneous murine astrocytoma have previously been fully characterised and found to differ in their degree of astrocytic differentiation. The in vitro chemosensitivity of the three lines has been investigated using the 35S-methionine uptake assay. Differential chemosensitivity was found to exist between the cell lines. The pattern of chemosensitivity in relation to astrocytic differentiation was complex but the least differentiated cell line, P497, tended to be the least chemosensitive.

In vivo characterization of a doxorubicin resistant B16 melanoma cell line

A doxorubicin-resistant line of B16 melanoma (B16VDXR) was obtained in vitro by continuous exposure to increasing concentrations of doxorubicin of an in vitro line (B16V) derived from the in vivo transplanted B16 melanoma. When injected s.c. into mice, B16VDXR exhibited histological features, metastatic behaviour, doubling time and tumourigenic potential similar to those of the parental B16V line. Tumours obtained by implantation of B16VDXR, however, had longer latency and permitted a longer survival time than B16V and had, as in vitro, a higher DNA content. After i.v. inoculation, B16VDXR cells had lower lung colonizing capability compared to B16V. B16V and B16VDXR had significantly lower metastatic potential compared to the B16 melanoma from which they derived. Doxorubicin treatment significantly delayed the growth of B16 and B16V transplanted s.c. and increased the life span of animals bearing B16V. B16VDXR was resistant to doxorubicin treatment when the in vitro resistance index was greater than 100. While the doxorubicin-resistance phenotype was stable in vitro for 50 passages, in vivo the resistance phenotype was lost in 5 passages and tumours grown from s.c. inocula of mixtures of similar percentages of sensitive and resistant cells behaved as sensitive tumours. Cis-diamminedichloroplatinum (II), although marginally active in animals bearing B16V, was highly effective in B16VDXR bearing animals, suggesting a collateral cis-diamminedichloroplatinum (II) sensitivity of the B16VDXR line. After a single i.v. administration, doxorubicin reached initially, in the B16VDXR line, levels similar to those found in the B16 and B16V lines, but its release was faster from the resistant line in comparison with the sensitive ones. Doxorubicin-resistance was not overcome by more frequent treatments with doxorubicin. This doxorubicin-resistant tumour line obtained in vitro and used as a first in vivo transplant, may be a suitable metastaizing model for in vivo study of the mechanisms of resistance and of collateral sensitivity and for screening new drugs.

Characterization of a doxorubicin-resistant murine melanoma line: studies on cross-resistance and its circumvention

A B16 mouse melanoma cell line resistant to doxorubicin was obtained by continuous in vitro exposure to the drug. The ID50 for this line was 200 times higher than that for the parental cell line. The resistant cell line had some biological characteristics similar to those of the sensitive parental cell line, like saturation density and protein content. Differences were found in doubling time which was longer, cloning efficiency which was lower and DNA content which was higher in the resistant as compared to the parental line. Intracellular distribution of doxorubicin was also different having a nuclear-cytoplasmic ratio higher in sensitive than in resistant cells. Melanin content was an unstable feature in the sensitive cell line, whereas melanin was always present in resistant cells. Resistance to doxorubicin was maintained during 50 in vitro passages in the absence of the drug. Cross-resistance was found with vincristine and other anthracyclines, like daunorubicin and 4'-epi-doxorubicin but not with cis-platinum, and a new doxorubicin derivative, 4'-deoxy-4'-iodio-doxorubicin. The B16 line showed a lower resistance index to 4'-deoxy-doxorubicin and 4-demethoxy-daunorubicin (30 and 3 respectively), as compared to doxorubicin. Doxorubicin-resistance was partially circumvented by pretreatment of resistant cells with verapamil, a calcium chelating agent, and by trifluoperazine, a calmodulin-antagonist.

Clonal variation in the sensitivity of a murine mammary carcinoma to melphalan

The sensitivity to melphalan of clones derived from individual lung colonies produced by i.v. injection of cells of the MT murine mammary carcinoma (caMT) and its melphalan-resistant sub-line (MTME16) has been examined. A degree of clonal heterogeneity was observed which was greater than could be explained by experimental variation. The distribution of melphalan sensitivities in both wild-type caMT and MTME16 raises questions as to the validity of a two-compartment model of drug-resistance development in tumours. A more complex model, possibly involving a continuous spectrum of drug sensitivity, is required. Differences in the sensitivity of the clonal lines of wild-type caMT in various passages were observed and this would appear to be due to phenotypic instability in these lines. This suggests that to use survival data from clones which have been passaged many times for predicting the response of the parent tumour may be misleading.

Emergence of nitrosourea resistant sublines of Lewis lung tumour following MeCCNU treatment in vivo

Several different drug retreatment protocols were employed to examine the emergence of resistance to MeCCNU in Lewis lung tumours. Previous studies suggested that although the majority of cells in untreated Lewis lung tumours were sensitive to MeCCNU, there was a very small proportion of resistant cells (approximately 0.001%) that limited "tumour cure' with that drug. If such cells were inherently drug resistant then it should be possible to derive highly resistant tumours by repeated drug treatment. In the first experiment tumours were treated with a single high dose of MeCCNU (35 or 40 mgkg-1) and on regrowth, transplanted into fresh mice and tested for drug sensitivity. Using both excision cell survival and growth delay endpoints, only approximately 25% of tumours were significantly resistant to the test dose, suggesting that many tumours resist the effects of the drug for reasons other than the presence of inherently drug resistant cells. One of the tumours (R4), that regrew after the initial treatment and appeared to be resistant to the test treatment, was retreated with a further 30 mgkg-1 MeCCNU and became more resistant. This line, designated R4/1, was cross-resistant to the other nitrosoureas, BCNU and CCNU, but not to cyclophosphamide, melphalan, cis-platinum or ionising radiation. The effect of treatment dose on the kinetics of MeCCNU resistance development was also studied in a retreatment regimen where the tumours were allowed to regrow and then transplanted into fresh hosts for the next treatment. Resistance developed more quickly at an intermediate dose of 15 mgkg-1 than at 7.5 mgkg-1 where the selective pressure was lower, or at 30 mgkg-1 where there was probably extinction of partially resistant cells. Resistance to MeCCNU developed even more quickly when tumours were retreated several times in the same host, although in a similar experiment with cyclophosphamide no resistance occurred.