Establishment of L1210 leukemia cells resistant to the distamycin-A derivative (FCE 24517): characterization and cross-resistance studies

Resistance to minor groove binders

In this paper multiple resistance mechanisms to minor groove binders (MGBs) are overviewed. MGBs with antitumor properties are natural products or their derivatives and, as expected, they are all substrates of P-glycoprotein (P-gp). However, a moderate expression of P-gp does not appear to reduce the sensitivity to trabectedin, the only MGB so far approved for clinical use. Resistance to this drug is often related to transcriptional mechanisms and to DNA repair pathways, particularly defects in transcription-coupled nucleotide excision repair (TC-NER). Therefore tumors resistant to trabectedin may become hypersensitive to UV rays and other DNA damaging agents acting in the major groove, such as Platinum (Pt) complexes. If this is confirmed in clinic, that will provide the rationale to combine trabectedin sequentially with Pt derivates.

DNA minor groove binding ligands: a new class of anticancer agents

This paper gives an overview of the available pharmacological and clinical data of a new class of anticancer drugs which act by binding DNA in the minor groove, comprising cyclopropylpyrrolo-indole (CC-1065) derivatives and Distamycin derivatives. The emphasis of this review is placed on the distinctive mode of action of these drugs. Molecular pharmacology studies indicate that CC-1065 and its derivatives, and the benzoyl mustard derivative of Distamycin, Tallimustine, possess the most striking DNA sequence specificity of alkylation observed to date for an alkylating agent of relatively small molecular weight. The effects on the regulation of gene transcription, the perturbation of the cell cycle, and the mechanism involved in the repair of the DNA lesions induced by these drugs all strongly support the view that minor groove ligands act by a mechanism different from those previously described for other anticancer drugs. The CC-1065 derivatives, Adozelesin and its prodrug, Carzelesin, and Tallimustine were found to be very effective against several murine tumours and human xenografts, and were shown to be active against experimental tumours that were resistant to other antineoplastic agents, including conventional alkylating agents. The clinical studies performed so far do not confirm in humans the remarkable antitumour activity observed in mice. The major reason appears to be a very high susceptibility of human bone marrow to both CC-1065 derivatives and Tallimustine as compared to mouse bone marrow, which makes it impossible to administer these drugs at sufficiently high doses to exert antitumour effects. The search for new minor groove binders, which possess a different DNA sequence specificity of alkylation and which are less cytotoxic for human bone marrow cells, is still in progress. It will be several years before it is possible to draw firm conclusions on the clinical effectiveness of this class of drugs.

Development of distamycin-related DNA binding anticancer drugs

The relatively low therapeutic index of the clinically used alkylating agents is probably related to the fact that these compounds cause DNA damage in a relatively unspecific manner, mainly involving guanine-cytosine rich stretches of DNA present in virtually all genes, therefore inducing unselective growth inhibition and death, both in neoplastic and in highly proliferative normal tissues. These considerations explain why in the last twenty years there has been an increasing interest in the identification of compounds which can target DNA with a much higher degree of sequence specificity than that of conventional alkylators. Minor groove binders (MGBs) are one of the most widely studied class of alkylating agents characterised by a high level of sequence specificity. The prototype of this class of drugs is distamycin A which is an antiviral compound able to interact, non-covalently, in theminor groove of DNA in A-T rich regions. It is not cytotoxic against tumour cells and thus has been used as a carrier for targeting cytotoxic alkylating moieties in theminor groove of DNA. The benzoyl mustard derivative of distamycin A, tallimustine, was found to be able to alkylate the N(3) of adenine in theminor groove of DNA only in the target hexamer 5'-TTTTGA or 5'-TTTTAA. Tallimustine was investigated in the clinic and was not successful because it causes severe bone marrow toxicity. The screening of other distamycin derivatives, which maintain antitumour activity and exhibit much lower toxicity against human bone marrow cells than tallimustine led to the identification of brostallicin (PNU-166196) which is currently under early clinical investigation. Although MGBs which bind DNA in A-T rich regions have not fulfilled the expectations, it is too early to draw definitive conclusions on this class of compounds. The peculiar bone-marrow toxicity observed in the clinic both with tallimustine or with CC-1065 derivatives is not necessarily a feature of all MGBs, as indicated by recent evidence obtained with brostallicin and other structurally unrelated MGBs (e.g., ET-743).

Resolution of a CPzI precursor, synthesis and biological evaluation of (+) and (-)-N-Boc-CPzI: a further validation of the relationship between chemical solvolytic stability and cytotoxicity

The chemical resolution, using N-tosyl-L-proline as a chiral auxiliary, of a racemate of the pyrazole analog (+/-)-N-Boc-CPzI of the left hand segment (CPI) of the antitumor agent CC-1065, and the cytotoxic evaluation of both enantiomers are described. The reported results further validate the direct relationship between chemical solvolytic stability of the cyclopropane ring and cytotoxicity proposed by Boger and coworkers.

Tallimustine lesions in cellular DNA are AT sequence-specific but not region-specific

Tallimustine (FCE 24517) is an AT-specific alkylating antitumor derivative of distamycin. This study examined levels of tallimustine lesions in intracellular DNA, their sequence- and region-specificity, and the long-range distribution of the drug binding motif. Tallimustine adducts in DNA converted to strand breaks by heating allowed the quantitation of drug lesions. In bulk DNA of intact human leukemia CEM cells, tallimustine formed 0.15 +/- 0.04 and 0.64 +/- 0.18 lesions/kbp at 5 and 50 microM, respectively. These lesions represent monoadducts as no interstrand cross-links or DNA-protein cross-links were detected. Tallimustine adducts in intracellularly treated DNA showed a general preference for sequences with T-tracts, suggesting a propensity for intrinsically bent motifs. Major drug-adducted sites identified by repetitive primer extension, included 5'-TTTTGPu-3' and 5'-TTTTGC-3' motif. Despite the high specificity at the nucleotide level, tallimustine did not differentiate among bulk DNA and three discrete AT-rich regions of genomic DNA examined by quantitative PCR stop assay with lesion frequencies ranging from 0.23 to 0.39 lesions/kbp at 25 microM drug. In comparisons of lesion frequencies and cytotoxicity, tallimustine adducts are approximately 50 times more lethal than relatively nonsequence specific cisplatin adducts but are >100 times less lethal than lesions by an unrelated AT-specific drug, bizelesin. However, the 5'-TTTTGPu-3' motifs targeted by tallimustine are relatively infrequent and scattered throughout the genome. In contrast, the motifs 5'-T(A/T)(4)A-3' motifs targeted by bizelesin, while also infrequent, cluster in defined AT-rich islands. The lack of region-specificity may be the reason tallimustine adducts, despite high AT-specificity at the nucleotide level, are less lethal than region-specific bizelesin adducts.

Alpha-bromoacryloyl derivative of distamycin A (PNU 151807): a new non-covalent minor groove DNA binder with antineoplastic activity

PNU 151807 is a new synthetic alpha-bromoacryloyl derivative of distamycin A. In the present study we investigated the DNA interaction and the mechanism of action of this compound in parallel with the distamycin alkylating derivative, tallimustine. PNU 151807 possesses a good cytotoxic activity in in vitro growing cancer cells, even superior to that found for tallimustine. By footprinting experiments we found that PNU 151807 and tallimustine interact non-covalently with the same AT-rich DNA regions. However, differently from tallimustine, PNU 151807 failed to produce any DNA alkylation as assessed by Taq stop assay and N3 or N7-adenine alkylation assay in different DNA sequences. PNU 151807, like tallimustine, is able to induce an activation of p53, and consequently of p21 and BAX in a human ovarian cancer cell line (A2780) expressing wild-type p53. However, disruption of p53 function by HPV16-E6 does not significantly modify the cytotoxic activity of the compound. Flow cytometric analysis of cells treated with equitoxic concentrations of PNU 151807 and tallimustine showed a similar induction of accumulation of cells in the G2 phase of the cell cycle but with a different time course. When tested against recombinant proteins, only the compound PNU 151807 (and not tallimustine or distamycin A) is able to abolish the in vitro kinase activity of CDK2-cyclin A, CDK2-cyclin E and cdc2-cyclin B complexes. The results obtained showed that PNU 151807 seems to have a mechanism of action completely different from that of its parent compound tallimustine, possibly involving the inhibition of cyclin-dependent kinases activity, and clearly indicate PNU 151807 as a new non-covalent minor groove binder with cytotoxic activity against cancer cells.

Structure-activity relationship of a series of C-terminus modified aminoalkyl, diaminoalkyl- and anilino-containing analogues of the benzoic acid mustard distamycin derivative tallimustine: synthesis, DNA binding and cytotoxicity studies

As part of our investigations into the design of more cytotoxic analogues of the experimental anticancer drug tallimustine, 1, C-terminus modified aminoalkyl-, 2a-c, diaminoalkyl-, 3, and anilino-containing, 4, derivatives have been synthesized. Compounds 2a-c differ by 2, 3, or 4 methylene units in the C-terminus, respectively. Results from an ethidium displacement study on poly(dA-dT), poly(dG-dC), calf thymus DNA and T4 coliphage DNA showed that compounds 2-4 interact in the minor groove of the polynucleotides with a preference for poly(dA-dT) over poly(dG-dC). Compound 4 bound more weakly to the DNAs than 2a-c and 3. Using a CD dilution assay compounds 2a-c and 3 were demonstrated to bind irreversibly to calf thymus DNA. The sequence selectivity by which compounds 2-4 alkylate DNA was demonstrated using a Taq polymerase stop assay. All the compounds alkylated preferentially at the 3'-purine residue in a 5'-TTTTGPu-3' sequence (Pu = A or G). This observed sequence specificity is similar to that of tallimustine and a related compound 5. At an equimolar concentration the aminoalkyl compounds 2a-c (2b > 2a > 2c), and diaminoalkyl compound 3 were more efficient at alkylating these sequences than the anilino compound 4. Following a one hour exposure of human chronic myeloid leukemia K562 cells, compounds 2b and 3 have lower IC50 values (1.64 microM and 3.03 microM, respectively) than tallimustine (5 microM) and similar values to a related compound 5 (2.2 microM). The order of cytotoxicity for all the compounds is 2b > 5 > 3 > 2a > 1 > 2c = 4. These results indicate that the cytotoxicities of these compounds are related to their relative ability to alkylate the consensus DNA binding sequence.

Tallimustine in advanced previously untreated colorectal cancer, a phase II study

Tallimustine is a novel benzoyl mustard derivative from distamycin A with a unique mode of action. It is a DNA minor groove binder and produces highly sequence-specific alkylations. Previous studies have shown significant anti-tumour effects in animal models. We performed a phase II study in previously untreated patients with advanced colorectal cancer, using a schedule of i.v. bolus infusions of 900 microgram m-2 once every 4 weeks. Seventeen patients were enrolled, and no responses were documented in 14 evaluable patients. Toxicity mainly consisted a highly selective neutropenia, which warrants further investigation of this agent in combination with myeloid growth factors.

Decreased tyrosine phosphorylation in tumour cells resistant to FCE 24517 (tallimustine)

Resistance to FCE 24517 is not related to the emergence of any of the most frequently observed phenotypes. We have found that two resistant cell lines (L1210/24517 murine leukaemia and LoVo/24517 human colon adenocarcinoma) present congenital modifications in tyrosyl phosphatase and kinase activities. Moreover, the cytotoxic activity of FCE 24517 is increased in combination with a tyrosine phosphatase inhibitor and decreased in combination with protein kinase inhibitors, this being in agreement with the hypothesis that the activity of this drug is strictly dependent on the presence of tyrosine phosphorylated protein(s).

Comparison of cell-cycle phase perturbations induced by the DNA-minor-groove alkylator tallimustine and by melphalan in the SW626 cell line

Tallimustine or N-deformyl-N-[4-N-N,N-bis(2-chloroethylamino)benzoyl], a distamycin-A derivative (FCE 24517), is a novel anti-cancer agent which alkylates N3 adenine in the minor groove of DNA. The cell-cycle phase perturbations induced by the drug were investigated and compared with those caused by melphalan (L-PAM) in SW626 human ovarian-cancer cells. By coupling bromodeoxyuridine (BUdR) immunoreaction with biparametric flow-cytometric (FCM) analysis, we investigated the cell-cycle phase perturbation induced by tallimustine or L-PAM, considering separately the cells which, during the 1-hr treatment, were in the S phase or in G1-G2/M phases of the cell cycle. L-PAM delayed the S-phase progression of cells exposed to the drug when they were in S phase, with a consequent accumulation of cells as soon as they reached the G2 phase. In contrast, the S-phase cells treated with tallimustine were not perturbed during the DNA-synthesis phase progression, and were blocked in G2 only after they had passed through the G1/S transition of a new cell cycle. In cells which were in G1 or G2/M phases during drug treatment, tallimustine and L-PAM caused similar accumulation in G2. The differences in the cell-cycle perturbation caused by tallimustine and L-PAM may well be related to the different DNA damage the 2 drugs produced. These findings emphasize the different properties of DNA-minor-groove alkylating agents and conventional ones.

Phase I study of the novel distamycin derivative tallimustine (FCE 24517)

BACKGROUND

Tallimustine, a benzoyl nitrogen mustard derivative of the antiviral agent distamycin A, is a new alkylating agent which binds to A-T rich regions of DNA in the minor groove producing highly sequence-specific alkylations. Its main preclinical features are a significant antitumor activity in animal models and a lack of cross-resistance in vitro and in vivo with L-PAM. Myelotoxicity was dose-limiting in animals, with a more than 100-fold difference in bone marrow sensitivity between mice and dogs.

PATIENTS AND METHODS

Forty adult patients (pts) with solid malignancies were entered in the study. The drug was administered as an IV bolus every 4 weeks. CBC was repeated twice a week and serial assessments of renal function were performed in the week following the first cycle. From the starting dose of 50 micrograms/m2, corresponding to 1/3 of the highest non-toxic dose (HNTD) in dogs, there were increases through 10 dose levels, with reliance only on the features of the myelotoxicity observed.

RESULTS

The main toxic effect was neutropenia which was dose-limiting, selective and short-lasting. Only previously-untreated pts received doses of 750 micrograms/m2 or more, with grade 4 neutropenia occurring in > or = 75% of the cycles. The maximally tolerable dose (MTD) was defined as 1250 micrograms/m2, with 3 of 3 pts developing febrile neutropenia requiring IV antibiotics. A platelet count of < 100 x 10(3)/microliters was observed in only one pt. Bone marrow aspiration performed in selected pts on days 8 and 15 confirmed a highly selective impairment by tallimustine of the myeloid lineage, with rapid recovery of the proliferative compartment. Pharmacokinetic studies performed at 1000 micrograms/m2 and 1250 micrograms/m2 showed a rapid fall of the plasma levels within the first 2 hours with drug concentrations between 100 ng/ml and 400 ng/ml within the first hour. A partial response of 4 months' duration was reported in one previously-untreated pt with cutaneous recurrences of malignant mesothelioma.

CONCLUSIONS

The report of some antitumour efficacy, the high selectivity of neutropenia, the lack of significant non-hematological toxic effects and the occurrence of detectable but still low plasma drug concentrations suggest that further clinical evaluation of higher doses of tallimustine in combination with colony-stimulating factors would be justified.

Characterisation of a LoVo subline resistant to a benzoyl mustard derivative of distamycin A (FCE 24517)

Human colon adenocarcinoma cells (LoVo) resistant to the new antitumor agent FCE 24517 [benzoyl-mustard derivative of distamycin A] (LoVo/24517) are resistant to the selecting agent and related molecules as well as to vinblastine, with marginal or no resistance to other antitumour drugs. Treatment with verapamil, tamoxifen, nicergoline or cyclosporin A only partially restores the activity of FCE 24517 against LoVo/24517 cells. Such results suggest that resistance mechanisms possible specific for this class of compounds are operating.