Acridine-O⁶-benzylguanine hybrids: Synthesis, DNA binding, MGMT inhibition and antiproliferative activity

O⁶-Methylguanine-DNA-methyltransferase (MGMT) is a key DNA repair enzyme involved in chemoresistance to DNA-alkylating anti-cancer drugs such as Temozolomide (TMZ) through direct repair of drug-induced O⁶-methylguanine residues in DNA. MGMT substrate analogues, such as O⁶-benzylguanine (BG), efficiently inactivate MGMT in vitro and in cells; however, these drugs failed to reach the clinic due to adverse side effects. Here, we designed hybrid drugs combining a BG residue covalently linked to a DNA-interacting moiety (6-chloro-2-methoxy-9-aminoacridine). Specifically, two series of hybrids, encompassing three compounds each, were obtained by varying the position of the attachment point of BG (N⁹ of guanine vs. the benzyl group) and the length and nature of the linker. UV/vis absorption and fluorescence data indicate that all six hybrids adopt an intramolecularly stacked conformation in aqueous solutions in a wide range of temperatures. All hybrids interact with double-stranded DNA, as clearly evidenced by spectrophotometric titrations, without intercalation of the acridine ring and do not induce thermal stabilization of the duplex. All hybrids, as well as the reference DNA intercalator (6-chloro-2-methoxy-9-aminoacridine 8), irreversibly inhibit MGMT in vitro with variable efficiency, comparable to that of BG. In a multidrug-resistant glioblastoma cell line T98G, benzyl-linked hybrids 7a-c and the N⁹-linked hybrid 19b are moderately cytotoxic (GI₅₀ ≥ 15 μM after 96 h), while N⁹-linked hybrids 19a and 19c are strongly cytotoxic (GI₅₀ = 1-2 μM), similarly to acridine 8 (GI₅₀ = 0.6 μM). Among all compounds, hybrids 19a and 19c, similarly to BG, display synergic cytotoxic effect upon co-treatment with subtoxic doses of TMZ, with combination index (CI) values as low as 0.2-0.3. In agreement with in vitro results, compound 19a inactivates cellular MGMT but, unlike BG, does not induce significant levels of DNA damage, either alone or in combination with TMZ, as indicated by the results of γH2AX immunostaining experiments. Instead, and unlike BG, compound 19a alone induces significant apoptosis of T98G cells, which is not further increased in a combination with TMZ. These results indicate that molecular mechanisms underlying the cytotoxicity of 19a and its combination with TMZ are distinct from that of BG. The strongly synergic properties of this combination represent an interesting therapeutic opportunity in treating TMZ-resistant cancers.