Replacement of a Thiourea with an Amidine Group in a Monofunctional Platinum–Acridine Antitumor Agent. Effect on DNA Interactions, DNA Adduct Recognition and Repair

Abstract

A combination of biophysical, biochemical, and computational techniques was used to delineate mechanistic differences between the platinum–acridine hybrid agent [PtCl(en)(L)](NO3)2 (complex 1, en = ethane-1,2-diamine, L = 1-[2-(acridin-9-ylamino)ethyl]-1,3- dimethylthiourea) and a considerably more potent second-generation analogue containing L′ = N-[2-(acridin-9-ylamino)ethyl]-Nmethylpropionamidine (complex 2). Calculations at the density functional theory level provide a rationale for the binding preference of both complexes for guanine-N7 and the relatively high level of adenine adducts observed for compound 1. A significant rate enhancement is observed for binding of the amidine-based complex 2 with DNA compared with the thiourea-based prototype 1. Studies conducted with chemical probes and on the bending and unwinding of model duplex DNA suggest that adducts of complex 2 perturb B-form DNA more severely than complex 1, however, without denaturing the double strand and significantly less than cisplatin. Circular and linear dichroism spectroscopies and viscosity measurements suggest that subtle differences exist between the intercalation modes and adduct geometries of the two complexes. The adducts formed by complex 2 most efficiently inhibit transcription of the damaged DNA by RNA polymerase II. Not only do complexes 1 and 2 cause less distortion to DNA than cisplatin, they also do not compromise the thermodynamic stability of the modified duplex. This leads to a decreased or negligible affinity of HMG domain proteins for the adducts formed by either Pt-acridine complex. In a DNA repair synthesis assay the lesions formed by complex 2 were repaired less efficiently than those formed by complex 1. These significant differences in DNA adduct formation, structure, and recognition between the two acridine complexes and cisplatin help to elucidate why compound 2 is highly active in cisplatin-resistant, repair proficient cancer cell lines.

Synthesis, Aqueous Reactivity, and Biological Evaluation of Carboxylic Acid Ester-Functionalized Platinum–Acridine Hybrid Anticancer Agents

“…This value is in excellent agreement with the unwinding produced by compound 3 in site-specifically modified model duplexes. 14 Furthermore, the data demonstrate that compounds 9 and 11 produce essentially the same degree of unwinding as the parent compound (16±1°), suggesting that extension of the linker by one methylene group or addition of a dangling ester group have no effect on the intercalation geometry in the hybrid adducts. Quantitation of platinum bound to the DNA samples after 24 h of incubation and subsequent removal of unbound agent by microdialysis shows that only ~64% of compound 11 forms permanent adducts with the plasmid, whereas binding of the parent compound 3´ is quantitative under the same conditions (see Supporting Information).…”

“…11, 12 Compound 3 produces significantly higher intracellular concentrations and DNA adduct levels in NCI-H460 cells than cisplatin, 13 and the hybrid adduct itself proves to be a significantly more severe form of DNA damage than the common bifunctional cross-link. 14 The cumulative effect of these pharmacological parameters renders the hybrid agent an efficient inhibitor of DNA replication and inducer of cell death. 15 …”

Redesigning the DNA‐Targeted Chromophore in Platinum–Acridine Anticancer Agents: A Structure–Activity Relationship Study

“…All models of platinum complexes and transition states were built, optimized, and calculated according to a previously published procedure. [7] …”

“…[6] In addition, the hybrid adducts are significantly more potent inhibitors of RNA polymerase II (Pol II)-mediated transcription than the cross-links. [7] Fluorescent post-labeling in whole lung cancer cells recently revealed high levels of DNA adducts in all phases of the cell cycle, both in the loosely packaged and the condensed chromatin. [8] The cumulative effects of this damage result in cell cycle arrest in late G1/early S phase and efficiently trigger apoptosis in NSCLC models representing a diverse range of genetic backgrounds.…”

Transition Metal Intercalators as Anticancer Agents—Recent Advances

“…These lesions inhibit DNA synthesis through stalled replication forks and DNA double-strand breaks [35]. Furthermore, these adducts inhibit RNA polymerase II-mediated transcription more prominently than compared to cross-links by cisplatin [36]. …”