microRNAs as drug targets
Saadia Basharat, PhD, Senior Consultant
microRNAs (miRNA) are small noncoding RNAs that regulate the expression of multiple target genes by either blocking the translation of their target mRNAs (silencing them) or causing direct degradation of them. miRNAs play important roles in several biological processes such as immune modulation, metabolic control, neuronal development, cell cycle, muscle differentiation, and stem cell differentiation. Expression of certain miRNAs is altered in different diseases; for instance, mir-122 in hepatitis C, mir-33a in metabolic disease, mir-155 in inflammatory disease, mir-10b in glioblastoma, mir-33 in atherosclerosis, and mir-21 in hepatocellular carcinoma. The ability of miRNAs to target multiple mRNAs that are altered in disease conditions makes these molecules interesting therapeutic targets, and there are three main approaches to do so: expression vectors (also called miRNA sponges), small-molecule inhibitors and antisense oligonucleotides (ASOs).
- miRNA sponges are plasmid constructs either transiently or stably transfected into mammalian cells containing multiple binding sites for a chosen miRNA gene. The binding sites serve as decoys for the targeted miRNA. miRNA sponges mimic the effects of miRNA inhibitors; the endogenous miRNA’s function is lost thereby reversing the suppression of endogenous mRNA target genes.
- Small molecule miRNA inhibitors act mainly through transcriptional regulation of targeted miRNAs rather than inhibition of mRNA target recognition by miRNAs.
- ASOs, particularly those that target miRNAs directly (anti-miRs) to specifically inhibit miRNA function are an interesting approach. Anti-miRs bind to miRNA-induced silencing complexes (miRISCs), thereby blocking their binding to endogenous mRNA targets.
The history of miRNA drug development has been largely unsuccessful, and it has been more than a decade since the first miRNA-targeted drug entered the clinic. Unfortunately, progress since then has been sparse. Santaris’ Miravirsen being developed against mir-122 to prevent hepatitis C virus replication in the liver was first into the clinic, however it did not successfully pass phase II clinical trial. In 2016, Mirna Therapeutics stopped a phase I clinical trial of its miRNA mimic, MRX-34 in cancer patients because of reports of severe immune reactions. And just last year, Regulus Therapeutics discontinued development of mir-122-targeting RG-101 following reports of jaundice in several patients.
These drug failures represent the inherent difficulties in developing miRNA-targeting drugs. For instance, although miRNA sponges have been widely used to investigate miRNA function in vitro, their utility in vivo has so far been limited to transgenic animals in which the sponge mRNA is overexpressed in target tissues. The therapeutic potential of the small molecule approach is rather limited owing to their high EC50 values and the lack of information on direct targets. Naked oligonucleotides are incapable of penetrating negatively charged cell membranes and require modification or encapsulation to enable their entry into the cell interior. Chemical modification of oligonucleotides is required to increase their resistance to serum nucleases in order to enhance binding affinity for targeted miRNAs. Inefficient anti-miR delivery in vivo is still a problem; most chemically modified anti-miRs demonstrate inadequate tissue distribution unless they are administered with a carrier, and they are usually taken up by the liver and kidney and rapidly expelled through urine. Furthermore, the dose required for in vivo inhibition is quite high, which of course increases the likelihood of off-target effects.
Regardless of these failures, miRNAs remain to be attractive drug targets in many diseases. Many still believe in miRNAs and especially miRNA-targeting oligonucleotides, since they possess many advantages over the small-molecule approach. The most obvious is the ease with which oligonucleotides can be chemically modified to enhance their PK/PD profiles, and the ability of miRNAs to target multiple genes simultaneously – this is not something that small molecules can compete with. A few companies still developing programs in this space are:
- Regulus Therapeutics; even after its RG-101 failure, Regulus recently took on development responsibility for RG-012, a miR-21-targeting oligonucleotide in phase II clinical development for Alport syndrome;
- Miragen has three clinical-stage drug candidates; cobomarsen, remlarsen and MRG-110 in development for hematologic cancers, fibrosis and heart failure, respectively;
- Cardior Pharmaceuticals is developing a miRNA-targeting oligonucleotide for heart failure, currently in phase Ib clinical development, shortly after demonstrating a strong preclinical dataset including a study in 200 pigs. Cardior’s lead molecule targets mir-132 which regulates the remodelling of cardiac tissue that follows a heart attack.
Indeed, only a small number of miRNA therapeutics have moved in to clinical development since miRNAs were discovered. While substantial challenges remain in this area, including identification of the best miRNA targets for drug development, and the design of delivery vehicles that allow better drug stability and tissue-specific targeting, the genomics and proteomics tools now available to researchers, coupled with novel delivery technologies will enable miRNA-targeted therapeutics to become a viable option for the treatment of many diseases.