Synthesis and evaluation of new steroidal lactam conjugates with aniline mustards as potential antileukemic therapeutics

Alkylating agents are still nowadays one of the most important classes of cytotoxic drugs, which display a wide range of therapeutic use for the treatment of various cancers. We have synthesized and tested four hybrid homo-azasteroidal alkylating esters for antileukemic activity against five sensitive to alkylating agents human leukemia cell lines in vitro and against P388 murine leukemia in vivo. Comparatively, melphalan and 3-(4-(bis(2-chloroethyl)amino)phenoxy)propanoic acid (POPAM) were also examined. All the homo-aza-steroidal alkylators showed relatively lower acute toxicity, very promising and antileukemic activity both in vitro and in vivo.

Studies on the nitroreductase prodrug-activating system. Crystal structures of complexes with the inhibitor dicoumarol and dinitrobenzamide prodrugs and of the enzyme active form

The E. coli nitroreductase enzyme (NTR) has been widely used in suicide gene therapy (GDEPT and ADEPT) applications as a activating enzyme for nitroaromatic prodrugs of the dinitrobenzamide class. NTR has been previously shown to be a homodimeric enzyme with two active sites. We present here the crystal structures of the reduced form of NTR and its complexes with the inhibitor dicoumarol and three dinitrobenzamide prodrugs. Comparison of the structures of the oxidized and reduced forms of the native enzyme shows that the principal structural changes occur in the FMN cofactor and indicate that the enzyme itself is a relatively rigid structure that primarily provides a rigid structural framework on which hydride transfer occurs. The aziridinyldinitrobenzamide prodrug CB 1954 binds in nonidentical ways in both of the two active sites of the homodimeric enzyme, employing both hydrophobic and (in active site B) a direct H-bond contact to the side chain of Lys14. In active site A the 2-nitro group stacks above the FMN, and in active site B the 4-nitro group does, explaining why reduction of either nitro group is observed. In contrast, the larger mustard group of the dinitrobenzamide mustard compound SN 23862 forces the prodrug to bind at both active sites with only the 2-nitro group able to participate in hydride transfer from the FMN, explaining why only the 2-hydroxylamine reduction product is observed. In each site, the nitro groups of the prodrug make direct H-bond contacts with the enzyme; in active Site A the 2-nitro to Ser40 and the 4-nitro to Asn71, while in active Site B the 2-nitro contacts the main chain nitrogen of Thr41 and the 4-nitro group the Lys14 side chain. The related amide-substituted mustard SN 27217 binds in a broadly similar fashion, but with the larger amide group substituent able to reach and contact the side chain of Arg107, further restricting the prodrug conformations in the binding site. The inhibitor dicoumarol appears to bind primarily by pi-stacking interactions and hydrophobic contacts, with no conformational changes in the enzyme. One of the hydroxycoumarin subunits stacks above the plane of the FMN via pi-overlap with the isoalloxazine ring, penetrating deep into the groove, with the other less well-defined. These studies suggest guidelines for further prodrug design. Steric bulk (e.g., mustard rather than aziridine) on the ring can limit the possible binding orientations, and the reducible nitro group must be located para to the mustard. Substitution on the carboxamide side chain still allows the prodrugs to bind, but also limits their orientation in the binding site. Finally, modulating substrate specificity by alteration of the structure of the enzyme rather than the prodrug might usefully focus on modifying the Phe124 residue and those surrounding it.

Self-immolative nitrogen mustards prodrugs cleavable by carboxypeptidase G2 (CPG2) showing large cytotoxicity differentials in GDEPT

Nineteen novel potential self-immolative prodrugs and their corresponding drugs have been synthesized for gene-directed enzyme prodrug therapy (GDEPT) with carboxypeptidase G2 (CPG2) as the activating enzyme. The compounds are derived from o- and p-amino and p-methylamino aniline nitrogen mustards. Their aqueous stability, kinetics of drug release by CPG2, and cytotoxicity in the colon carcinoma cell line WiDr, expressing either surface-tethered CPG2 (stCPG2(Q)3) or control beta-galactosidase, are assessed. The effect of various structural features on stability, kinetics of activation, and biological activity is discussed. The p-methylamino prodrugs are the most stable compounds from this series, with the largest cytotoxicity differentials between CPG2-expressing and nonexpressing cells. The most potent compounds in all series are prodrugs of bis-iodo nitrogen mustards. 4-[N-[4'-Bis(2' '-iodoethyl)aminophenyl]-N'-methylcarbamoyloxymethyl]phenylcarbamoyl-l-glutamic acid, compound 39b, is 124-fold more cytotoxic to WiDr cells expressing CPG2 than to cells expressing beta-galactosidase. An additional six compounds show better cytotoxicity differential than the published N-[4-[(2-chloroethyl)(2-mesyloxyethyl)amino]benzoyl]-l-glutamic acid (CMDA) prodrug.

Efficient clearance of poly(ethylene glycol)-modified immunoenzyme with anti-PEG monoclonal antibody for prodrug cancer therapy

The F(ab')(2) fragment of the anti-TAG-72 antibody, B72.3, was covalently linked to Escherichia coli-derived beta-glucuronidase that was modified with methoxypoly(ethylene glycol). The conjugate (B72.3-betaG-PEG) localized to a peak concentration in LS174T xenografts within 48 h after injection, but enzyme activity persisted in plasma such that prodrug administration had to be delayed for at least 4 days to avoid systemic prodrug activation and associated toxicity. Conjugate levels in tumors decreased to 36% of peak levels at this time. Intravenous administration of AGP3, an IgM mAb against methoxypoly(ethylene glycol), accelerated clearance of conjugate from serum and increased the tumor/blood ratio of B72. 3-betaG-PEG from 3.9 to 29.6 without significantly decreasing the accumulation of conjugate in tumors. Treatment of nude mice bearing established human colon adenocarcinoma xenografts with B72. 3-betaG-PEG followed 48 h later with AGP3 and a glucuronide prodrug of p-hydroxyaniline mustard significantly (p< or =0.0005) delayed tumor growth with minimal toxicity compared to therapy with a control conjugate or conventional chemotherapy.

Macromolecular derivatives of N,N-di-(2-chloroethyl)-4-phenylene diamine mustard. 2. In vitro cytotoxicity and in vivo anticancer efficacy

Prodrugs of N,N-di-(2-chloroethyl)-4-phenylene diamine (PDM) based on soluble poly[N5-(2-hydroxyethyl)-l-glutamine] (PHEG) have been evaluated as tumour-targeted drugs. These materials are designed to exploit the enhanced permeability of tumour vasculature, combining a passive tumour tropism with systemic liberation of free PDM. Modification of PDM by coupling via oligopeptide spacers onto a polymeric carrier significantly reduced its cytotoxicity towards different cell types in vitro. On the other hand, incubation of the cells with the PHEG-Gly-Phe-Ala-Leu-PDM conjugate in the presence of collagenase IV led to the release of lethal amounts of free drug, resulting in higher cytotoxicity for this derivative. The PHEG-Gly-Phe-Ala-Leu-PDM conjugate, which is rapidly degraded by lysosomal and tumour-associated enzymes also showed a decreased systemic toxicity in vivo and could be administered at a dose of 8 mg PDM/kg body weight intravenously, compared with just 2 mg/kg for free PDM. Furthermore, this derivative also showed better antitumour activity against a C26 colorectal carcinoma tumour model, compared with no activity for the free drug. The results indicate that the PHEG-Gly-Phe-Ala-Leu-PDM conjugate is a promising candidate for cancer treatment.

Aniline mustard analogues of the DNA-intercalating agent amsacrine: DNA interaction and biological activity

Two series of analogues of the clinical antileukemic drug and DNA-intercalating ligand amsacrine have been prepared, containing aniline mustard sidechains of varying reactivity, linked either at the 4-position of the intercalating acridine chromophore (type A) or at the 1'-position of the 9-anilino group (type B). DNase I footprinting assays showed that compounds of type B had stronger reversible binding to DNA than did compounds of type A. Compounds of each type showed similar patterns of alkylation-induced cleavage of DNA, and alkylate at the N7 of guanines in runs of guanines (similar to the pattern for untargeted mustards) as well as some adenines. Both classes of compounds crosslinked DNA, although those bearing relatively inactive mustards did so only at high drug/base pair ratios. However, while the patterns of DNA alkylation were broadly similar, the compounds were considerably more cytotoxic than analogous untargeted mustards. Comparison of their cytotoxicities in wild-type and DNA repair-deficient lines indicated this toxicity was due to DNA crosslinks (except for the least reactive SO2-linked mustards). The 4-linked analogues showed slightly higher in vivo antileukemic activity than the corresponding 1'-linked analogues.

Synthesis and biological activity of DNA damaging agents that form decoy binding sites for the estrogen receptor

It is a goal of cancer chemotherapy to achieve the selective killing of tumor cells while minimizing toxicity to normal tissues. We describe the design of selective toxins forming DNA adducts that attract the estrogen receptor (ER), a transcription factor that is overexpressed in many human breast and ovarian tumors. The compounds consist of 4-(3-aminopropyl)-N,N-(2-chloroethyl)-aniline linked to 2-(4'-hydroxyphenyl)-3-methyl-5-hydroxy-indole. The former moiety is a DNA damaging nitrogen mustard and the latter is a ligand for the ER. The connection between these groups was refined to permit DNA adducts formed by the mustard portion of the molecule to present the ligand domain so that it was able to interact efficiently with the ER. By using 16-mers containing specific DNA adducts, it was determined that monoadducts and putative intrastrand crosslinks were preferred targets for the ER over interstrand crosslinks. A series of structurally related 2-phenylindole mustards was prepared, some of which were selectively toxic to the ER-positive breast cancer cell line MCF-7, as compared with the ER(-) negative line MDA-MB231. The ability both to bind to DNA and to interact significantly with the ER were essential to achieve selective lethality toward ER(+) cells. Compounds forming DNA adducts without the ability to bind receptor showed similar toxicities in the two cell lines. Several models could explain the selective toxicity of the mustard-phenylindole compounds toward ER(+) cells. The favored model suggests that a mustard-DNA adduct is shielded by the ER from DNA repair enzymes and hence cells possessing an abundance of the ER selectively retain the adduct and are killed.

Hypoxia-selective antitumor agents. 14. Synthesis and hypoxic cell cytotoxicity of regioisomers of the hypoxia-selective cytotoxin 5-[N,N-bis(2-chloroethyl)amino]-2,4-dinitrobenzamide

A series of regioisomers of the novel hypoxia-selective cytotoxin (HSC) 5-[N,N-bis(2-chloroethyl)-amino]-2,4-dinitrobenzamide (2a) have been prepared by displacement of the chloro group from methyl chlorodinitrobenzoates or the corresponding carboxamides with diethanolamine, followed by dimesylation and mesylate displacement with LiCl. The compounds fall into two classes, where the two nitro groups have either a meta or an ortho (or para) disposition to each other. The four meta derivatives had one-electron reduction potentials in the range -340 to -375 mV, similar to that of the known isomer 2a, while the other isomers had much higher values (-262 to -285 mV). The meta derivatives were much less cytotoxic to AA8 cells under aerobic conditions (IC50s from 75 to 470 microM) than were the other compounds (IC50s from 1.6 to 20 microM). However, the ratios of IC50s of the compounds in repair-proficient (AA8) and repair-deficient (UV4) cell lines varied, indicating differing contributions of DNA alkylation to aerobic toxicity between the isomers, with no clear relationship between this and nitro group disposition. The hypoxic selectivities of the (dimethylamino)ethylcarboxamide analogues for each isomer were determined by clonogenic assay against both AA8 and UV4 cells. With one exception, the meta derivatives showed excellent hypoxic selectivities (ca. 45-115-fold) against UV4 cells, while the ortho or para isomers had little selectivity (ca. 2-7-fold). A possible reason may be that the latter compounds, with higher reduction potentials, undergo rapid bioreduction even under aerobic conditions. None showed hypoxic selectivities greater than 2-3-fold against AA8 cells. The 3-[N,N-bis(2-chloroethyl)amino]-2,6-dinitrobenzamide isomer (5b), which showed the highest hypoxic selectivity for UV4 cells in this series, was active against both hypoxic and aerobic cells in KHT tumors in mice at well-tolerated doses, and showed superior in vivo activity to the previously studied 2,4-dinitro isomer 2b.

DNA-directed aniline mustards with high selectivity for adenine or guanine bases: mutagenesis in a variety of Salmonella typhimurium strains differing in DNA-repair capability

Two closely-related aniline monomustards (1 and 2), linked to a DNA-targeting acridine chromophore by a linker chain of different length, show high selectivity for alkylation of polymer DNA. The shorter-chain derivative (2) alkylates mainly at guanine N7 sites, while the longer-chain analogue (1) reacts almost exclusively at adenine N1. The biological effects of these compounds have been studied in standard Ames Salmonella typhimurium strains in order to determine the mutagenic consequences of such well-defined DNA lesions, and the effect of DNA-repair systems on them. Both compounds caused detectable mutations in strains TA1537, TA98 or TA100 and some related strains. Mutation rates were greatly enhanced in strains carrying either a uvrB deletion or the plasmid pKM101. Frameshift mutagenesis by both compounds was completely eliminated by recA deletion, in both the presence or absence of the plasmid. The adenine-selective compound (1) appeared more sensitive to the DNA-repair defects than the guanine-selective derivative (2). Additionally, only the adenine-selective compound (1) caused statistically significant levels of detectable mutation in the repair-proficient strains TA102, TA4001 or TA4006. The bacterial mutagenesis evidence suggests that a bulky, major groove-residing adenine lesion may be more readily recognised by DNA-repair systems, and more likely to lead to a wider range of mutagenic events, than a similar guanine lesion.

Identification of the major lesion from the reaction of an acridine-targeted aniline mustard with DNA as an adenine N1 adduct

DNA adducts of two acridine-linked aniline half-mustards have been isolated and identified. The compound where the half-mustard is attached to the DNA-targeting acridine moiety by a short linker chain alkylates both double- and single-stranded DNA exclusively at guanine N7, as do the majority of known aromatic and aliphatic nitrogen mustards. The longer-chain analogue, also containing a more reactive half-mustard, shows a strikingly different pattern, alkylating double-stranded DNA to yield primarily (> 90%) the adenine N1 adduct, together with < 10% of the adenine N3 adduct and only trace amounts of the guanine N7 adduct. In the presence of MgCl2 (which is known not to inhibit the interaction of drugs at minor groove sites), the adenine N3 adduct is the major product. The latter compound is the first known aniline mustard (and apparently the first known alkylating agent of any type) to preferentially alkylate adenine at the N1 position in duplex DNA. These results are consistent with previous work [Prakash et al. (1990) Biochemistry 29, 9799-9807], which showed that the preferred site of DNA alkylation by the corresponding long-chain acridine-linked aniline bis-mustards in general was at major groove sites of adenines and identifies the major site of alkylation as adenine N1 and not N7. This selectivity for adenine N1 alkylation is suggested to result from a preference for the acridine mustard side chain of these compounds to project into the major groove following intercalation of the acridine, coupled with structural distortion of the DNA helix to make the N1 positions of adenines adjacent to the intercalation sites more accessible.

Comparative study on the mutagenicity of three structurally related substituted aniline mustards in the Salmonella/microsome assay

The ortho, meta and para isomers of N,N-bis(2-chloroethyl)aminocinnamic acid were tested for their ability to mutate Salmonella typhimurium strains in the Salmonella/microsome mutagenicity test. The aim of the work was to establish a structure-activity relationship between these three isomers. The drugs were found to induce base-pair substitutions, causing dose-dependent increases in his+ revertants, in strains TA100 and TA1535. The study showed that the position of the substituent groups influenced the mutagenic activity of the compounds. The ortho isomer exhibited a poorer mutagenic effect than meta and this was found to be a weaker mutagen than para. The presence of metabolic activation enzymes in the test system induced a further increase in his+ revertants, in strains TA100 and TA1535, which is consistent with the findings for melphalan, a cancer chemotherapeutic agent with a chemical structure similar to that of the isomers tested.

DNA-directed aniline mustards based on 9-aminoacridine: interaction with DNA

A series of 4-substituted aniline mustards ArNH(CH2)nOpC6H4N(CH2CH2Cl)2, where Ar is an acridine and n varies from 2 to 5, interact with DNA. Scatchard analysis shows the compounds bind tightly, with a binding site size similar to that of 9-aminoacridine. The rate of hydrolysis of the mustards, measured by HPLC, is essentially constant across the series. With increasing length of the polymethylene linker, non-covalent binding becomes less strong, but the rate of DNA alkylation increases. Viscometric helix extension measurements and electrophoretic analyses using closed circular supercoiled DNA show that all the compounds are DNA intercalating ligands. Despite these similarities, the compounds are known to have quite different patterns of DNA alkylation, switching from guanine to adenine alkylation as the chain length is extended.

Modulation of mutagenic properties in a series of DNA-directed alkylating agents by variation of chain length and alkylator reactivity

Four series of aniline mustards linked to a DNA-affinic acridine chromophore by alkyl chains of varying length (2-5 carbon atoms) have been studied for their mutagenic properties, as estimated in four strains of Salmonella typhimurium and in Saccharomyces cerevisiae strain D5. The four series have very different mustard reactivities, as determined by the aniline link group (-O-, -CH2-, -S- or -SO2-). Some of the derived compounds cause frameshift mutagenesis which can be detected in TA98 and also "petite" mutagenesis activity, neither of which occur to significant extents with the parent mustards or with 9-aminoacridine. None of the derived compounds are as effective as the parent mustards in mitotic crossing-over, nor do they show ability for frameshift mutagenesis in S. typhimurium TA1977 which is typical of acridines. Some of the compounds have comparable frameshift activity to compounds such as ICR-191, but appear to have a different base-pair preference. The results indicate clear structure-activity relationships for the spectrum of mutagenic activity, which relate to both chain length and alkylator reactivity, for these compounds.

In vitro and in vivo activities of monoclonal antibody-alkaline phosphatase conjugates in combination with phenol mustard phosphate

The prodrug p-[N,N-bis(2-chloroethyl)amino]phenyl phosphate (phenol mustard phosphate, POMP) was prepared from p-[N,N-bis(2-chloroethyl)amino]phenol (phenol mustard, POM) by phosphorylation with phosphoryl chloride, followed by aqueous hydrolysis. It was found that POMP was much less cytotoxic than POM when tested against H2981 human lung and H3396 human breast carcinoma cells in vitro. Pretreatment of the H2981 cells with L6-alkaline phosphatase (L6-AP), a monoclonal antibody conjugate that could bind to cell surface antigens, greatly enhanced the cytotoxic effects of POMP in an immunologically specific manner. Owing to its reduced toxicity in nude mice, larger amounts of POMP compared to POM could be administered. Neither agent exhibited significant in vivo antitumor activity when tested against subcutaneous H2981 tumors in nude mice. However, antitumor activity was observed in animals receiving L6-AP 48 h prior to POMP administration. This level of activity was greater than with the drugs alone, or a combination of 1F5-AP (nonbinding control) with POMP.

Hybrid anticancer compounds. Steroidal lactam esters of carboxylic derivatives of N,N-bis (2-chloroethyl) aniline (review)

For the rational design of more specific alkylating agents, we suggested new biological platforms able to deliver the alkylating moieties to specific target site and on the other hand we hoped to lead in compounds with synergistic activity. As biological platforms have been used steroidal lactams of A and D- ring and as alkylating agents carboxylic derivatives of N,N-bis (2-Chloroethyl) aniline which combine to the steroid by an easily cleaved ester bond. These homo-aza-steroidal esters gave satisfactory results in early and advanced P388, L1210 leukemias and solid tumors. Whereas unmodified steroidal esters have generally been reported to be inactive in treatment of L1210 leukemia. The steric arrangement of the alkylating moiety greatly effects toxicity and activity of the drugs, while the steric arrangement of the hydrogen atom at position 5 influences these parameters. Isosterism of alkylating agent is the factor for biological action. The amide group of the lactam molecule may be essential for activity.

DNA-directed alkylating agents. 3. Structure-activity relationships for acridine-linked aniline mustards: consequences of varying the length of the linker chain

Four series of acridine-linked aniline mustards have been prepared and evaluated for in vitro cytotoxicity, in vivo antitumor activity, and DNA cross-linking ability. The anilines were attached to the DNA-intercalating acridine chromophores by link groups (-O-, -CH2-, -S-, and -SO2-) of widely varying electronic properties, providing four series of widely differing mustard reactivity where the alkyl chain linking the acridine and mustard moieties was varied from two to five carbons. Relationships were sought between chain length and biological properties. Within each series, increasing the chain length did not alter the reactivity of the alkylating moiety but did appear to position it differently on the DNA, since cross-linking ability (measured by agarose gel assay) altered with chain length, being maximal with the C4 analogue. The in vivo antitumor activities of the compounds depended to some extent on the reactivity of the mustard, with the least reactive SO2 compounds being inactive. However, DNA-targeting did appear to allow the use of less reactive mustards, since the S-linked acridine mustards showed significant activity whereas the parent S-mustard did not. Within each active series, the most active compound was the C4 homologue, suggesting some relationship between activity and extent of DNA alkylation.