STP705 Shows Promise in Cutaneous Squamous Cell Carcinoma

Mark Nestor, MD, PhD

STP705 met the primary efficacy endpoint in an ongoing phase II study of patients with cutaneous squamous cell carcinoma in situ (isSCC), according to topline results reported by Sirnaomics Inc., the developer of the small interfering RNA (siRNA) therapeutic.

An interim analysis of the first 3 cohorts in the trial showed that treatment with STP705 led to complete histological clearance of isSCC in two-thirds of patients.

Overall, 10 of the 15 evaluable patients achieved complete histological clearance. In 2 of the 3 cohorts—the 10- and 20-µg cohorts—histological clearance of lesion occurred in 3 of the 5 patients. In the third cohort—those treated at a 30-µg dose—4 of 5 patients achieved histological clearance of lesion, with a dose-dependent manner.

"The standard of care for the treatment of squamous cell carcinoma in situ is either surgical excision or destruction or radiation therapy. The advent of a nonsurgical, scarless option using a simple series of injections with high cure rates will change the paradigm for the treatment of this common non melanoma skin cancer," Mark Nestor, MD, PhD, principal investigator of the phase II trial, and co-director of the Center for Clinical and Cosmetic Research, stated in a press release from Sirnaomics.

In the press release, Sirnaomics explained that "STP705, is an siRNA therapeutic that takes advantage of a dual-targeted inhibitory property and polypeptide nanoparticle—enhanced delivery to directly knock down both TGF-β1 and COX-2 gene expression."

Overall, the open-label, dose-escalation phase II study enrolled 5 patients in each of 5 cohorts with doses of STP705 ranging from 10 μg to 120 μg. Treatment was administered intravenously once weekly for up to 6 weeks. The cutoff date was March 2, 2020, for the interim analysis of the first 3 cohorts (10, 20, and 30 μg). Analysis is ongoing of the final 2 cohorts, which are administering STP705 at 60- and 120-µg doses, respectively.

"The results of this study are very encouraging based on the fact that we were able to demonstrate high rates of histological clearance, which is the gold standard for skin cancer. This was combined with an excellent safety profile and most importantly, a lack of local cutaneous skin reactions, which is very important to both patients and clinicians as they look for alternatives to surgical excision of these lesions," Michael Molyneaux, MD, chief medical officer of Sirnaomics, said in the press release.

According to Sirnaomics, no treatment-related adverse events (AEs) or serious AEs have been reported thus far. The company also noted that there have been no reported cases of significant cutaneous skin reactions.

"This interim clinical readout not only demonstrates the potential of RNAi therapeutics in the oncology application, but also illustrates the potential safety and efficacy of polypeptide nanoparticle delivery for siRNA therapeutics. In addition, the profound therapeutic efficacy further validates TGF-β1 and COX-2 as an important cancer drug target and our dual-targeting strategy for novel RNAi cancer therapeutics," Patrick Lu, PhD, president and CEO of Sirnaomics, stated in the press release.

STP705 has been granted FDA Orphan Drug Designations for the treatment of patients with cholangiocarcinoma, primary sclerosing cholangitis, and hepatocellular carcinoma. The designation is intended to expedite the review and development of treatments for rare diseases, defined as those affecting fewer than 200,000 people in the United States.

Sirnaomics Announces Positive Topline Results from Interim Analysis of Ongoing Phase II Clinical Trial Evaluating STP705 in Cutaneous Squamous Cell Carcinoma in situ (isSCC). Published April 9, 2020. https://bit.ly/2Vjbv8f. Accessed April 9, 2020.