Out of the lab and into patients: targeting APE1/Ref-1

How does the dysregulation of APE1/Ref-1 contribute to the aggressive nature of cancers such as PDAC, MPNST, colon, and other cancers?

We have been studying a redox signalling protein called Redox effector factor-1/Apurinic Endonuclease 1(APE1/Ref-1 or just Ref-1) and its role in the regulation of numerous transcription factors (TFs) such as HIF-1a, NFκB, STAT3, AP-1 (Fos/Jun) and others. Regulation of HIF-1a by Ref-1 along with these other TFs contributes to the metabolic rewiring and gene expression changes that are observed in many cancers.

Ref-1 is both a redox (reduction-oxidation) signalling protein and a DNA repair enzyme. Ref-1 acts as a redox factor that stimulates the DNA binding activity of numerous TFs, by converting the TFs from an oxidised-inactive to reduced-active form and enabling full TF activation. These TFs are often dysregulated in multiple cancers including PDAC, MPNST, bladder, colon and paediatric cancers such as sarcomas. Therefore, blockade of Ref-1 redox activity can impede upon many cancer-related pathways.

What makes APE1/Ref-1 a promising target for cancer therapy, and how do small molecules like APX3330 selectively inhibit its redox function?

As mentioned, Ref-1 protein is elevated in several cancers and other diseases impacting multiple signalling pathways which are involved in the cancer and disease progression. APX3330, that we developed, as well as new second generation compounds that we are developing, block the ability of Ref-1 to turn on the downstream TFs. Keeping the TFs turned down, or off, results in the blockage of the pathways that are driving the cancer and other diseases we have been studying.

The parent compound APX3330 was developed and advanced from our academic lab and licensed to Apexian Pharmaceuticals to conduct Phase I clinical trials in Oncology (NCT03375086). It was subsequently licensed to Ocuphire Pharma, a public company who did Phase II trials in Diabetic Retinopathy (DR) and Diabetic Macular Edema (DME) (NCT04692688). Ocuphire completed Phase II trials in DR in 2023, had a successful EOP2 meeting with FDA and will conduct a Phase II/III trial in DR in 2025. In the Phase I oncology clinical trial APX3330 demonstrated excellent safety and pharmacokinetic / pharmacodynamic (PK/PD) profile, target engagement with no overt toxicity (NCT03375086). Additionally, six of 19 patients had disease stabilisation for > 12 weeks and 4 of 19 for ≥ 36 weeks. Evaluation of tissue biopsies indicated reduced levels of transcription activity for regulators of cancer survival pathways regulated by Ref-1 including HIF1a, NFkB, and STAT3, indicating that APX3330 mediates the redox activity of the Ref-1 protein target as expected.

What are the potential benefits of targeting APE1/Ref-1’s redox function in treating diabetic retinopathy (DR) compared to current anti-VEGF therapies?

DR manifests itself through increased inflammation and angiogenesis in the retina, two areas regulated by the TFs Ref-1 control and APX3330 blocks. DR is the leading cause of blindness in working age adults, impacting approximately 10 million patients in the US. Most patients have early-stage disease (non-proliferative diabetic retinopathy), which is generally untreated. Current therapies are invasive and typically used for advanced DR, and do not address multiple disease pathways. APX3330 may represent a promising oral option for slowing DR progression by inhibiting Ref-1, which will potentially block unwanted angiogenesis and inflammation.

Conversely, what are the potential challenges of targeting APE1/Ref-1’s redox function in treating diabetic retinopathy (DR)?

The major challenge is assuring enough of the inhibitor drugs, given orally, can reach the retina in the eye to have an impact. However, preclinical studies as well as results from the very first phase II trial performed by Ocuphire show positive results suggesting APX3330 can reach the retina as needed. Fewer APX3330-treated patients had DR worsening versus the placebo group. This demonstrated efficacy on the FDA-confirmed endpoint of ≥ 3 step worsening on the binocular DRSS person-level scale. Also, fewer APX3330-treated subjects developed proliferative diabetic retinopathy (advanced DR) compared to the placebo patients.

What other diseases do you envision could be addressed by targeting the Ref-1 protein with your inhibitors?

We currently have some very exciting data in preclinical models of Inflammatory Bowel Disease (IBD) (Crohn’s and Ulcerative Colitis) and necrotizing enterocolitis (NEC), a devastating neonatal surgical emergency caused by an upregulation of oxidative stressors that leads to inflammation and bowel injury in neonates.

In the IBD models, systemic treatment with APX3330 reduced inflammation, protected against decreasing body weight, reduced rectal prolapse, edema and observed bleeding, corrected colonic contractile activity and intestinal permeability. Also, what is very novel is we observed protection of nerve fibers and glial cells in the colon. Therefore, we observed a unique and multifactorial protection approach.

For NEC, our recent preclinical data demonstrates a potential use of Ref-1 inhibitors, showing a reduction of inflammation and bowel injuries.

Please see the link to Dr Kelley’s talk here: https://aaps2024.eventscribe.net/ajaxcalls/PresentationInfo.asp?PresentationID=1455522