Structure Guided Design, Synthesis, and Biological Evaluation of Oxetane-Containing Indole Analogues

Abstract

The oxetane functional group offers a variety of potential advantages when incorporated within appropriate therapeutic agents as a ketone surrogate. OXi8006, a 2-aryl-3-aroyl-indole analogue, functions as a small-molecule inhibitor of tubulin polymerization that has a dual mechanism of action as both an antiproliferative agent and a tumor-selective vascular disrupting agent. Replacement of the bridging ketone moiety in OXi8006 with an oxetane functional group has expanded structure activity relationship (SAR) knowledge and provided insights regarding oxetane incorporation within this class of molecules. A new synthetic method using an oxetane-containing tertiary alcohol subjected to Lewis acid catalyzed conditions led to successful Friedel-Crafts alkylation and yielded fourteen new oxetane-containing indole-based molecules. This synthetic approach represents the first method to successfully install an oxetane ring at the 3-position of a 2-aryl-indole system. Several analogues showed potent cytotoxicity (micromolar GI₅₀ values) against human breast cancer cell lines (MCF-7 and MDA-MB-231) and a pancreatic cancer cell line (PANC-1), although they proved to be ineffective as inhibitors of tubulin polymerization. Molecular docking studies comparing colchicine with the OXi8006-oxetane analogue 5m provided a rationale for the differential interaction of these molecules with the colchicine site on the tubulin heterodimer.

Keywords: Friedel-Crafts alkylation; Inhibition of tubulin polymerization; Oxetane.

Figure 1.

Natural Products Colchicine, CA-4, CA-1 and Selected Synthetic Small-Molecule Analogues Designed and Synthesized by the Pinney Group^,–,– as Inhibitors of Tubulin Polymerization.

Figure 2.

Molecular Design Paradigm: Oxetane Moiety as Ketone Surrogate in OXi8006

Figure 3:. Scratch Assay for PANC-1 Cells Treated with Compound 5c

Representative images of PANC-1 cells at 1, 24, and 40 h after making scratches. Cells were incubated at 37 °C with 5% CO₂. A(1) A(2) and A(3): control wells; scratch gap closed after 40 h; B(1), B(2) and B(3): 0.5 μM 5c treated cells; cells slowly move towards the center of the gap, but gap is not completely closed after 40 h; C(1), C(2) and C(3): 1 μM 5c treated cells; gap remains open after 40 h.

Figure 4:. Scratch Assay for PANC-1 Cells Treated with Compound 5h

Representative images of PANC-1 cells at time 1, 24, and 48 h after making scratches. Cells were incubated at 37 °C with 5% CO₂. A(1) A(2) and A(3): control wells; scratch gap closed after 48 h; B(1), B(2) and B(3): 0.5 μM 5h treated cells; cells slowly move towards the center of the gap, but gap is not closed after 48 h; C(1), C(2) and C(3): 1 μM 5h treated cells; gap remains open after 48 h.

Figure 5:. Compound 5m Trimethoxy Ring (blue) Interaction with the α-Subunit Residues (Blue).

The DAMA-colchicine (co-crystalized structure with tubulin) trimethoxy ring indicated in yellow interaction with β-tubulin residues (green).

Figure 6:

Molecular Docking of Compound 5m, α-Subunit Residues (Blue); All Other Residues are Located on the β-Subunit (Green).

Scheme 1.

Synthesis of Indole-Based Oxetane-Containing Analogues

Scheme 2.

Synthesis of Oxetane-Substituted OXi8006