As a proximate mediator of the splicopathy that characterizes DM1, DMPK RNA carrying the toxic expanded CUG repeat sequence represents a potentially important target for therapy development. A candidate therapeutic with drug-like properties, including the ability to access the many affected target tissues in DM1, that specifically targets expanded repeat RNA, would represent an important compound for evaluation in interventional clinical trials. A new study has taken steps to establish preclinical proof of concept for such an agent.

Initial Development of Cugamycin

Ms. Alicia Angelbello (PhD graduate student) and Dr. Matt Disney at The Scripps Research Institute, Florida and their colleagues (at University of Florida and Iowa State) have developed a small molecule compound capable of cleaving DMPK expanded repeat RNA, as evidenced by data from studies in DM1 patient-derived myotubes and a DM1 mouse model (Angelbello et al., 2019). The candidate therapeutic has been named Cugamycin, because of its specificity/affinity for expanded, but not sub-disease threshold, CUG repeats.

The research team explored the idea that bringing specificity to a known RNA cleaving compound, bleomycin A5, represents a rationale path toward a DM1 therapeutic. By attaching bleomycin A5 to an RNA-binding molecule, the team could achieve a critical level of specificity and thereby advance both bioactivity/efficacy and safety of the resulting compound—Cugamycin. Target recognition and cleavage specificity of Cugamycin was established through studies showing in vitro binding and efficient cleavage of CUG^exp, but not DNA repeat hairpins.

Cell- and Animal-Based Support for Further Development of Cugamycin

In vitro analyses using DM1 patient-derived and control myotubes showed that Cugamycin was cell-permeable (overcoming a hurdle seen in large molecule development for DM1 muscle) and tracks to the nucleus. DMPK RNA cleavage efficiency in this model was 40%, with an EC₂₅ in high nanomolar range—notably, wild type DMPK RNA was spared. The research team also established that Cugamycin reduced nuclear foci and rescued defects in the splicing events tested in the cell model. Direct comparison of Cugamycin and antisense oligonucleotides (AONs) targeted to DMPK expanded repeat RNA showed higher specificity for the small molecule drug (AON sequence/chemistry used here failed to distinguish between expanded and subthreshold DMPK RNA).

Findings of subsequent in vivo evaluations of drug metabolism and pharmacokinetics (the other DMPK) of Cugamycin supported a move to mouse efficacy testing. Short-term dosing of 10 mg/kg ip every other day in adult HSA^LR mice was well tolerated and without side effects linked to the base bleomycin A5 compound. Short-term dosing also produced a 40% reduction in toxic DMPK RNA and reversal of splicing defects tested in hindlimb muscles, confirming systemic drug bioavailability and target engagement and modulation. Treated HSA^LR mice showed restoration of Clcn1 protein levels and reductions in myotonia. The high selectivity of the candidate therapeutic was confirmed in RNA-seq analyses of skeletal muscle samples from untreated wild type and vehicle only and Cugamycin treated HSA^LR mice.

The authors conclude that, as a preclinical candidate molecule, Cugamycin has limited liabilities, but do note the opportunities for further chemical analoging around its scaffold to continue to optimize and arrive at a development candidate for IND-enabling studies and entry into clinical trials.

Path Forward

Taken together, this new study provides a compelling case that small molecules can be developed to safely and effectively target toxic RNA cleavage in DM1. At the time of this writing, yet another reminder became available that efficacy in a mouse model may or may not translate into an effective drug in patients (Chakradhar, 2019 and justsaysinmice). Thus, while current findings are encouraging, they are in a model organism that cannot actually “have” the human disease—we look forward to further testing of Cugamycin and its analogs, and pursuit of all strategies for the development and regulatory approval of safe and effective drugs for patients living with DM.

References:

Precise small-molecule cleavage of an r(CUG) repeat expansion in a myotonic dystrophy mouse model.
Angelbello AJ, Rzuczek SG, Mckee KK, Chen JL, Olafson H, Cameron MD, Moss WN, Wang ET, Disney MD.
Proc Natl Acad Sci U S A. 2019 Mar 29. pii: 201901484. doi: 10.1073/pnas.1901484116. [Epub ahead of print]

It’s just in mice! This scientist is calling out hype in science reporting.
Chakradhar, S.
STAT. April 15, 2019