Targeting cGMP/PKG signaling for the treatment or prevention of colorectal cancer with novel sulindac derivatives lacking cyclooxygenase inhibitory activity

Approximately 28 million people in the United States have precancerous colonic adenomas with many at high risk of developing colorectal cancer who could benefit from a pharmacological approach to prevent malignant progression. Clinical, epidemiological, and preclinical studies have reported or provided mechanistic evidence that nonsteroidal anti-inflammatory drugs (NSAIDs) can reduce adenoma formation and the risk of developing colorectal cancer. Unfortunately, the long-term use of NSAIDs is not recommended because of potentially fatal toxicities resulting from cyclooxygenase (COX) inhibition and the depletion of physiologically important prostaglandins. However, multiple investigators have concluded that the mechanism responsible for the antineoplastic activity of NSAIDs does not require COX inhibition, which suggests the feasibility of developing safer and more efficacious derivatives for the treatment or prevention of colorectal cancer by targeting such mechanisms. We have widely reported that the NSAID, sulindac, inhibits certain phosphodiesterase (PDE) isozymes to increase intracellular cyclic guanosine monophosphate (cGMP) levels and activate protein kinase G (PKG) at concentrations that inhibit cancer cell growth in vitro. PDE5 and PDE10 are potential targets given that both are overexpressed in colon adenomas and adenocarcinomas and essential for colon cancer cell proliferation. An extensive medicinal chemistry campaign was conducted to identify novel sulindac derivatives lacking COX inhibitory activity with increased potency and selectivity to inhibit cancer cell growth. From a large collection of over 1500 compounds sharing the indene scaffold of sulindac, a series of derivatives were identified that inhibit cancer cell growth with high potency and selectivity by a mechanism involving the activation of cGMP/PKG signaling. Development candidates have been identified that block the oncogenic effects from APC gene mutations responsible for most human colorectal cancers by suppressing β-catenin-dependent transcriptional activity. This review describes the scientific rationale for the development of sulindac derivatives lacking COX inhibitory activity with improved potency and selectivity to inhibit PDE5 and/or PDE10 for the treatment or prevention of colorectal cancer.