General Research Area
Research in our laboratory is interdisciplinary in nature and spans the fields of organic and inorganic synthesis, medicinal chemistry, (bio)physical chemistry, and molecular and cellular biology. The ultimate goal of our research program is to design structurally unique organic–inorganic hybrid molecules as pharmacophores for tackling chemoresistant disease and as probes of biomolecular structure and function. While the major thrust of our current research efforts is to promote our hybrid agents into clinical trials, we are equally interested in developing this technology into a tool for probing and manipulating nucleic acid secondary structure in live cells.
DNA-Targeted Platinum–Acridine Anticancer Agents
Summary: This multifaceted research program aims to exploit unprecedented small-molecule–DNA interactions for the treatment of intractable cancers, such as lung cancer. During the past 10 years, our serendipitous discovery that a non-cross-linking platinating–intercalating hybrid agent is able to produce cytotoxicity similar or superior to cisplatin has developed into an exciting medicinal chemistry program. Since the discovery of the parent drug, PT-ACRAMTU, our rigorous biophysical and biochemical mechanistic work combined with classical structure–activity relationship studies has resulted in a technology that stands a realistic chance of providing a potent weapon against non-small-cell lung cancer (NSCLC). NSCLC is an aggressive form of the disease, which has proven resistant to conventional cytotoxic agents as well as the newer targeted therapies. This research program has turned a serendipitous discovery into a potent armamentarium against an intractable form of cancer.
Milestones: Discovery of a structurally unique cytotoxic platinum–acridine hybrid agent (“PT-ACRAMTU”) (2001); complete biophysical, biochemical, and high-resolution structural characterization of the DNA damage produced by PT-ACRAMTU, including a combined high-resolution NMR solution structural and MM/MD study of a site-specific guanine adduct (2002-2005); discovery of previously unknown DNA minor-groove adducts formed by PT-ACRAMTU (2005); discovery of a structural modification, as a result of systematic SAR studies, that resulted in a PT-ACRAMTU analogue 500-fold more potent than the clinical drug cisplatin and tumor growth inhibition in lung cancer xenografts in mice (2008); PT-Amidine, a second-generation platinum-acridine derivative, produces replication fork arrest and causes a pre-apoptotic cellular response different from that of clinical platinum drugs (2011).
- Molecular and cellular pharmacology: Defining the mechanisms by which platinum–acridines by-pass cellular defense mechanisms and effect apoptosis.
- Tumor-targeted delivery and release/activation: reducing the systemic toxicity and improving the efficacy of the platinum–acridines by:
- 1) Reducing the chemical promiscuity of the agents (pro-drug design),
- 2) Introducing the agents as (hyper)cytotoxic “warheads” in polymer- and (monoclonal) antibody-based (ADCs) delivery vehicles.
- 3) Targeting the agents to hormone-dependent cancers (e. g., breast) using drug conjugation strategies.
- Preclinical development: xenograft models, toxicity profiles, pharmacokinetic and pharmacodynamic (PK/PD) studies.
- Medicinal chemistry: development of novel chemical delivery and release mechanisms using click and conjugation chemistrie.
- Structural biology/biophysics/molecular biology: structural basis of cellular response, X-ray crystallographic study of drug-modified DNA and DNA–drug–enzyme ternary complexes, nucleotide excision and replication fork repair.
Platinum–Intercalator Conjugates Targeted at DNA and RNA G-Quadruplexes: Biological and Biomedical Applications
Summary: This project was inspired by our discovery that platination of DNA by PT-ACRAMTU results in the formation of a large percentage of minor-groove adducts at the adenine-N3 position, a mechanism unprecedented in platinum–DNA interactions (the naturally preferred target of clinical platinum drugs is the major-groove nucleophilic site, guanine-N7). We found that the concept of intercalator-driven platination can be used to not only generate DNA groove selective adducts, but can be also exploited to target non-classical DNA and RNA secondary structures.
Milestones: Discovery of kinetically favored platination of loop adenine residues by PT-ACRAMTU in the G-quadruplex form of the human telomeric repeat (2007); design of a perylene–platinum conjugate that recognizes the antiparallel form of the human telomeric G-quadruplex, which produces a higher binding affinity for telomeric DNA than for the RNA form (2011).
Future Directions: The structures and biophysical properties of G-quadruplexes formed by the sequence TTAGGG are being intensively investigated. While the existence in live cells and potential biological roles of the human telomeric quadruplex are still being debated, the damage response observed in cancer cells treated with quadruplex-binding/inducing ligands, indeed, is consistent with cell kill mechanisms triggered by G-quadruplex formation. The shortening of the telomeres with each replication cycle limits the life span of normal cells. By contrast, in the majority of tumors, the telomeres are efficiently restored due to overexpressed telomerase, the reverse transcriptase involved in telomere elongation, rendering cancer cells immortal. One way to intercept the unlimited proliferation of cancer cells, therefore, would be to inhibit telomerase-catalyzed strand elongation with small molecules that stabilize the G-quadruplex form of the DNA single-stranded terminus. The vast majority of G-quadruplex-binding molecules are reversible binders containing planar moieties that associate with the G-tetrad via p-stacking interactions, and comparatively few approaches of targeting the human telomeres with agents that form covalent adducts have been reported. This project will develop platinum–intercalator conjugates as G-quadruplex-specific agents. Specifically, the G-quadruplex-forming sequences of the telomeric DNA 3’ overhang and the hTERC RNA in telomerase will be targeted. The utility of these agents as irreversible G-quadruplex trapping agents for biological and therapeutic applications is being be explored.
Cationic Gold(I) Complexes Derived From PT-ACRAMTU
Summary: As an extension of our synthetic studies on platinum-acridines we have generated gold(I) analogues of PT-ACRAMTU (2009). These compounds are only moderately cytotoxic in cancer cell lines but were found to be potent and selective inhibitors of Mycobacterium tuberculosis in a high-throughput screen performed by the Tuberculosis Antimicrobial Acquisition and Coordinating Facility (TAACF) of the National Institute of Allergy and Infectious Diseases (NIAID). Future work on this project will establish SAR within this class of compounds and explore the possibility of turning these agents into cancer chemotherapies directed at the mitochondrial selenium-containing enzyme thioredoxin reductase, a potential target of gold-based anticancer agents.