Contraception, or birth control, is an important tool in family planning. Given the fourfold increase in population over the last century1 there is a clear need for more affordable, reversible, and safe methods of contraception. At present, the responsibility of taking contraceptives falls largely on people with female reproductive organs as there is no current method of birth control for people with male reproductive organs. The search for a non-hormonal, male birth control has been an elusive goal in the field of reproductive health.
Recently, a group of scientists from Baylor College of Medicine with contributions from Promega scientists identified a novel compound that 1) inhibits a specific kinase and 2) functions as a reversible male contraceptive. The kinase targeted in this study is the serine/threonine kinase 33 (STK33); a genetic knockout of this gene in male mice is known to cause sterility. The team published their work in Science and utilized a suite of approaches—including DNA-Encoded Libraries (DELs), crystallography, and cellular NanoBRET™ Target Engagement Kinase Assays—to discover a potent inhibitor of STK33 (CDD-2807). The CDD-2807 inhibitor has shown promising results in inducing reversible contraception in male mice, marking a significant milestone in the development of non-hormonal contraceptive options. Let’s dive into the foundation, novel methodology, collaboration, and implications for this work.
Background: From Target Determination to Target Efficacy
The research, led by Dr. Martin Matzuk (Baylor) with contributions from Promega R&D Scientists represents a groundbreaking expansion of kinase research into reproductive health. Kinases play crucial roles in various cellular processes—including metabolism, cell signaling and DNA repair—making them ideal targets for drug discovery. Despite their significance, only a small fraction of the 538 known human kinases have been explored for therapeutic purposes, with most research focusing on cancer treatment2. Expanding beyond cancer treatment, kinases represent a viable target for non-hormonal male contraception: by targeting a specific kinase, researchers can achieve precise control over biological processes, potentially minimizing side effects and improving efficacy.
One such kinase, the serine/threonine kinase 33 (STK33), is often called a “dark kinase” due to limited research on its function2. However, STK33 has been found to play a critical role in spermatogenesis. Recent studies have shown that it is expressed in the testis—specifically in pachytene spermatocytes and spermatids, which are stages crucial for sperm developmen3t. Furthermore, genetic studies in mice have shown that the absence of STK33 leads to defective sperm morphology and motility, resulting in male infertility4. Similarly, men with mutations in the STK33 gene exhibit infertility, mirroring the phenotypes observed in knockout mice. This research sheds light on the function of SKT33 kinase and lays the groundwork for Dr. Matzuk’s team: STK33 as a pivotal protein in male fertility and an attractive target for contraception.
Building on this foundational understanding of STK33’s role in male fertility, researchers have explored its potential as a contraceptive target. In this work, the team developed a small-molecule inhibitor, CDD-2807, which binds specifically to STK33 and blocks its activity. In animal studies, administration of CDD-2807 led to infertility without affecting the size or overall health of the testis. Furthermore, when the treatment was discontinued, the fertility of the male mice was restored, highlighting the potential of STK33 inhibition as a safe and reversible method of male contraception.
Innovative Methods and Meaningful Collaborations
Screening small molecules is a laborious task, but important for uncovering potential drug targets. To overcome this labor-intensive process, the research team used DNA-Encoded Libraries (DELs). DELs allow scientists to screen billions of potential small-molecule inhibitors efficiently. By tagging each molecule with a unique DNA sequence, researchers could rapidly identify and isolate compounds that bind to the target protein (STK33), from vast chemical libraries. This high-throughput screening method streamlines the drug discovery process, enabling the identification of potent and selective inhibitors with speed and accuracy.
Once the research team identified the compound/target pair, the next step was to determine the potency and selectivity of CDD-2807 by measuring how effectively it binds to STK33 in a cellular context. The NanoBRET™ Target Engagement Kinase Assays played a crucial role this characterization process. NanoBRET™ TE Assays measure the interaction between small molecules and their target proteins in live cells, providing researchers with quantitative affinity and occupancy data within the cellular context.
Researchers used the NanoBRET® TE K192 Kinase Selectivity System to ensure that CDD-2807 was highly selective for STK33 and did not significantly interact with other kinases in live cells. This system allows for live cell profiling of a compound’s selectivity across a panel of 192 full-length kinases. By confirming that CDD-2807 selectively binds to STK33 with minimal off-target interactions, the researchers validated its potential as a specific inhibitor in cells. This selectivity analysis during lead optimization phase is crucial for understanding unwanted interactions that could lead to side effects and allowing further compound optimization to ensure the inhibitor’s action is confined to the intended target.
The success of this research is a testament to the power of leadership and collaboration. The leadership of Dr. Martin Matzuk and his team at Baylor guided the overall direction of the research and drove the development of exciting new inhibitors for STK33. Dr. Matzuk’s team brought extensive knowledge in reproductive biology and kinase research, which was crucial for understanding the implications of targeting STK33. The research team at Promega provided expertise in the application of cellular NanoBRET™ Target Engagement Kinase assays, enabling the rapid screening and characterization of potential inhibitors. This collaborative effort exemplifies how academic and industry partnerships can lead to groundbreaking discoveries.
Dr. Martin Matzuk explains the importance of NanoBRETTM TE in their research.
Conclusions and Broader Impacts
The research on STK33 inhibition and the development of the small-molecule inhibitor CDD-2807 mark a significant step forward in the search for non-hormonal male contraception. By leveraging advanced technologies like DNA-Encoded Libraries (DELs) and NanoBRET™ Target Engagement Kinase Assays, researchers successfully identified and optimized a compound that produces a reversible contraceptive effect in male mice. This work opens new possibilities for male contraception, providing a promising alternative that could help balance contraceptive responsibilities across the population, regardless of biological sex. Additionally, it demonstrates the potential of targeting kinases in various aspects of human health, paving the way for further exploration of “dark kinases” and their roles in different biological processes. The success of this project sets a precedent for future interdisciplinary research initiatives, emphasizing the importance of collaborative efforts in advancing scientific discoveries that can have a meaningful impact on global reproductive health and beyond
If you’d like to learn more about the technical details of this work, check out the publication in Science. To learn more about the collaborative efforts in this research, check out this press release.
The NanoBRET® TE K192 Kinase Selectivity System used in this study is now available for use by any researcher. To learn more about this system and more, check out this page.
- Max Roser and Hannah Ritchie (2023) – “How has world population growth changed over time?” Published online at OurWorldInData.org. Retrieved from: ‘https://ourworldindata.org/population-growth-over-time’ [Online Resource] ↩︎
- M. E. Berginski et al., The dark kinase knowledgebase: an online compendium of knowledge and experimental results of understudied kinases. Nucleic. Acids Res. 49, D529–D535 (2021). ↩︎
- A. O. Mujica, B. Brauksiepe, S. Saaler-Reinhardt, S. Reuss, E. R. Schmidt, Differential expression pattern of the novel serine/threonine kinase, STK33, in mice and men. FEBS J. 272, 4884–4898 (2005) ↩︎
- L. R. Martins et al., Stk33 is required for spermatid differentiation and male fertility in mice. Dev. Biol. 433, 84–93 (2018) ↩︎
Anna Bennett
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