DNA interstrand cross-linking and in vivo antitumor activity of the extended pyrrolo[2,1-c][1,4]benzodiazepine dimer SG2057.

The pyrrolobenzodiazepines (PBDs) are naturally occurring antitumor antibiotics and a PBD dimer (SJG-136, SG2000) is in Phase II trials. SG2000 is a propyldioxy linked PBD dimer which binds sequence selectively in the minor groove of DNA forming DNA interstrand and intrastrand cross-linked adducts, and also mono-adducts depending on sequence. SG2057 is the corresponding dimer containing a pentyldioxy linkage. SG2057 has multilog differential in vitro cytotoxicity against a panel of human tumour cell lines with a mean GI(50) of 212 pM. The agent is highly efficient at producing DNA interstrand cross-links in cells which form rapidly and persist over a 48 h period. Significant antitumor activity was demonstrated in several human tumor xenograft models. Cures were obtained in a LOX-IMVI melanoma model following a single administration and dose-dependent activity, including regression responses, observed in SKOV-3 ovarian and HL-60 promyelocytic leukemia models following repeat dose schedules. In the advanced stage LS174T model, SG2057 administered either as a single dose, or in two repeat dose schedules, was superior to irinotecan. SG2057 is therefore a highly active antitumor agent, with more potent in vitro activity and superior in vivo activity to SG2000, warranting further development.

SG2285, a novel C2-aryl-substituted pyrrolobenzodiazepine dimer prodrug that cross-links DNA and exerts highly potent antitumor activity.

The pyrrolobenzodiazepines (PBD) are naturally occurring antitumor antibiotics, and a PBD dimer (SJG-136, SG2000) is in phase II trials. Many potent PBDs contain a C2-endo-exo unsaturated motif associated with the pyrrolo C-ring. The novel compound SG2202 is a PBD dimer containing this motif. SG2285 is a water-soluble prodrug of SG2202 in which two bisulfite groups inactivate the PBD N10-C11 imines. Once the bisulfites are eliminated, the imine moieties can bind covalently in the DNA minor groove, forming an interstrand cross-link. The mean in vitro cytotoxic potency of SG2285 against human tumor cell lines is GI(50) 20 pmol/L. SG2285 is highly efficient at producing DNA interstrand cross-links in cells, but they form more slowly than those produced by SG2202. Cellular sensitivity to SG2285 was primarily dependent on ERCC1 and homologous recombination repair. In primary B-cell chronic lymphocytic leukemia samples, the mean LD(50) was significantly lower than in normal age-matched B and T lymphocytes. Antitumor activity was shown in several human tumor xenograft models, including ovarian, non-small cell lung, prostate, pancreatic, and melanoma, with cures obtained in the latter model with a single dose. Further, in an advanced-stage colon model, SG2285 administered either as a single dose, or in two repeat dose schedules, was superior to irinotecan. Our findings define SG2285 as a highly active cytotoxic compound with antitumor properties desirable for further development.

Linker length modulates DNA cross-linking reactivity and cytotoxic potency of C8/C8' ether-linked C2-exo-unsaturated pyrrolo[2,1-c][1,4]benzodiazepine (PBD) dimers.

A C2/C2'-exo-unsaturated pyrrolo[2,1-c][1,4]benzodiazepine (PBD) dimer 4b (DRG-16) with a C8-O(CH2)nO-C8' diether linkage (n = 5) has been synthesized that shows markedly superior in vitro cytotoxic potency (e.g., >3400-fold in IGROV1 ovarian cells) and interstrand DNA cross-linking reactivity (>10-fold) compared to the shorter homologue 4a (SJG-136; n = 3). In contrast, for the C-ring unsubstituted series, the corresponding n = 5 dimer (3c) is generally less cytotoxic and has a lower interstrand cross-linking reactivity compared to its shorter n = 3 homologue (3a). Dimer 4b cross-links DNA with >10-fold efficiency compared to 4a, and also inhibits the activity of the restriction endonuclease BamH1 more efficiently than either 3a or 4a. The C2-exo-unsaturated PBD dimers 4a,b are not only more effective than their C-ring saturated counterparts in terms of induced DeltaTm shift, but they also exert this effect more rapidly. Thus, while 3a and 3c exert 68 and 35% of their maximum effect immediately upon interaction with DNA, this level increases to 76 and 97% for 4a and 4b, respectively. Molecular modeling shows a rank order of 4b (n = 5) > 4a (n = 3) > 3a (n = 3) > 3c (n = 5) in terms of binding energy toward duplexes containing embedded target 5'-GAT(1-2)C cross-link sequences, reflecting the superior fit of the C2-exo-unsaturated rather than saturated C-rings of the PBD dimers. A novel synthesis of core synthetic building blocks for PBD dimers via stepwise Mitsunobu reaction and nitration with Cu(NO3)2 is also reported.