MYC is among the most dysregulated genes across all human cancers (1). The human MYC gene was identified in 1983, and since then there have been over 30,000 articles published on the role of MYC in cancer. As a master transcriptional regulator responsible for aberrant gene expression in numerous cancers, tools to modulate MYC expression and protein function are needed to develop effective strategies for targeted anti-MYC therapies. Despite the high value of potential MYC-directed therapeutics, such compounds have proven challenging to realize, leading MYC to be labeled classically “undruggable.” Drugging MYC will require creative and novel solutions to this decades-old challenge.

In our recently published study, we introduce a library of MYC-targeted antisense oligonucleotides (MYCASOs). Through a combination of third generation ASO stabilizing chemical modifications (locked nucleic acids, LNA) and a unique library design that scans the entire coding region of the full-length mature human MYC mRNA transcript, we synthesized compounds that potently and selectively degrade MYC mRNA and decrease MYC protein levels. MYCASO treatment decreased viability and proliferation in four distinct MYC-driven cancer cell lines, and notably decreased tumor burden and improved survival in a murine MYC-driven tumor model.

The library we constructed consists of 23 different MYCASO sequences, and while lipid-based transfection of each MYCASO resulted in near uniform MYC knockdown, we observed that unformulated (gymnotic) delivery resulted in highly variable MYC knockdown, illustrating the significant role that cellular uptake plays in ASO activity. These results highlight that further investigation into the mechanisms of ASO uptake and trafficking are warranted to fully understand the factors that contribute to ASO activity. Understanding of these pathways may further aid in the development of selective ASO delivery systems for those sequences that are not effectively internalized on their own.

Using total mRNA sequencing, we investigated the global transcriptional effects of MYCASO treatment. While we observed profound effects on MYC and MYC-associated gene expression signatures specifically with MYCASO treatment, we also observed generalized ASO responses, including activation of interferon pathways. Inhibition of MYC, which acts as a regulator of immune-related genes and suppresses immune responses in general, is also expected to result in activation of immune stimulatory pathways including interferon responses (2). These two properties of MYCASOs may positively interact to ultimately enhance targeted anti-MYC effects of MYCASO treatment.

In a mouse model of MYC-induced hepatocelluar carcinoma (HCC), MYCASO treatment led to a significant loss of tumor burden with an intermittent, short-term dosing schedule. Clear loss of MYC expression was observed specifically in treated animals, with dramatic improvement in overall survival compared to animals treated with a non-targeted control ASO. Notably these experiments did not utilize sophisticated formulations or nanoparticle carriers, indicating that the hepatotropic nature of these oligonucleotides may be leveraged for HCC treatment. It will be interesting to assess MYCASOs capability to target and penetrate tumor types within other tissues.  

The first ASO approved for clinical use was fomivirsen, a treatment for cytomegalovirus retinitis (3). It was not until 2013 that another ASO therapeutic approved: mipomersen for the treatment of homozygous familial hypercholesterolemia (4). In the past five years, seven more ASO therapies have been approved for clinical use. While this is an exciting time for ASO technology with momentum driving ASO therapeutics into the clinic, work remains to realize this class of therapeutic as effective cancer medicines. As the SARS-CoV-2 pandemic created an urgent demand for rapid vaccine development that mRNA-based vaccine technology rose to meet, nucleic acid-based drugs may someday similarly change the course of cancer outcomes. We consider our library of MYCASOs as an important addition to a growing collection of anti-MYC tools, and look forward further studies that will continue to explore their potential as cancer therapeutics.

References

1. Kalkat M, Melo JD, Hickman KA, Lourenco C, Redel C, Resetca D, et al. MYC Deregulation in Primary Human Cancers. Genes-basel. 2017;8(6):151. 
2. Schlee M, Hölzel M, Bernard S, Mailhammer R, Schuhmacher M, Reschke J, et al. c‐MYC activation impairs the NF‐κB and the interferon response: Implications for the pathogenesis of Burkitt’s lymphoma. Int J Cancer. 2007;120(7):1387–95.
3. Marwick C. First "antisense" drug will treat CMV retinitis. JAMA. 1998;280(10):871.
4. Parham JS, Goldberg AC. Mipomersen and its use in familial hypercholesterolemia. Expert Opin. Pharmacother. 2019;20(2):127-31.