Are you tired of dieting or pounding the pavement like David Goggins just to shed a few pounds? Imagine a future medicine that could mimic the benefits of literally running for days.
Weight loss meds like Ozempic have been sending shockwaves through Hollywood and Wall Street. Advocacy by famous figures went viral on social media, causing supply shortages and more recently, questions on their safety emerged. The stonks of select pharma companies exploded, with Danish Novo Nordisk’s market capitalization surpassing the country’s GDP.
Is this first wave of drugs already the be-all, end-all? Side effects like loss of hard-earned gains and pooping pants lead to many users stopping treatment.
Meet SLU-PP-332, a simple small molecule melting fat and effectively mimicking marathon training in mice – all without setting a tiny paw on a treadmill, and without eating less. We will cover this molecule’s discovery, chemical synthesis, and pre-clinical efficacy. This will enrich your interdisciplinary knowledge and give you some practise for data interpretation. We will also explain how other exercise mimetics work and cover random facts, such as taking a closer look at alleged health benefits of red wine.
How obesity Drugs Work
So regardless how we feel about it, obesity is the problem for healthcare systems, and source of many problems. By 2030, nearly half of Americans will be obese – not to mention overweight.
Why even consider drugs for weight loss? We all know that exercise and diet regimens have very low compliance in reality. People just don’t stick to them even if they know they should. Also, some patients have co-morbidities that make them exercise-intolerant. It doesn’t matter how much you want to be David Goggins – if you have chronic heart failure, you can’t run for days.
Another issue: the emerging obesity drugs melt away body mass, but much of it is also muscle tissue. Drugs which can trigger fat loss, retain muscle and simulate exercise would be helpful for many people.
Obviously, with hundreds of millions of obese people, this is a massive long-term opportunity for pharmaceutical companies. This bullish outlook has resulted in strong investor interest in obesity drug developers, adding billions of dollars to their valuations.
The drugs behind this gold rush are GLP1 agonists. As mimetics of the incretin hormone GLP1, they stimulate insulin production and help manage blood sugar – this is especially key for diabetics. They also slow down movement of food in your stomach which can help patients feel fuller faster and curb hunger. Leave a comment if you want to learn more about these hyped obesity drugs in a future video – they have a massive history of research behind them. Today, we will instead check out the so-called exercise mimetics.
Science Behind Exercise Mimetics
PGC-1a is a key link between endurance exercise and physiological adaptation. Expressed in various tissues, this is the master regulator of creation of mitochondria – also known as the powerhouse of the cell – as well as other processes like glucose and lipid metabolism. Because it’s a coactivator, it interacts with other transcription factors to modulate the expression of certain genes. Looking at the example of detoxification of reactive oxygen species, we realize this gets into complex cellular biology territory. Due to this complexity, dysregulation of PGC-1 alpha disrupts physiological processes and contributes to many diseases.
Why is this relevant for exercise mimetics? Well, while various mimetics have different primary targets, most ultimately all trace back to PGC-1 alpha.
One rather famous molecule in this class is resveratrol. This polyphenol is present in many foods and wines, and it can trigger just about every effect under the sun. It likely indirectly activates the so-called SIRT1 protein, which in turn deacetylates PGC-1 alpha and ramps up beneficial activities. There’s a lot of literature on this if you want to check out the cellular biology.
Some of the first insights came from a 2006 study looking at daily intake of resveratrol in mice being fed a high-fat diet. If you are good at playing ‘spot the difference’, you will notice that fat and muscle tissues feature much denser mitochondria. It looks like these mice adapted to exercise they didn’t perform – thus, we’re calling such effects mimicry.
Shockingly, some human studies showed resveratrol actually blunted some aspects of training adaptation. Ironically, removal of reactive oxygen species by resveratrol might limit training-induced adaptations. This once more highlights that in biology, nothing is simple or black-or-white.
Based on the immature human data, the verdict on resveratrol is still open. If you check Wikipedia, you can see that no health benefits have clear evidence. Such lacking clinical data is a common theme for exercise mimetics in general, as they represent a new class of compounds.
Even big pharma companies dabbled in this space, with GSK paying 700 million dollars for a biotech working on a resveratrol formulation 15 years ago. They did not test it in obesity but rather haematological cancers. This proved to be bad luck as they killed the program after seeing increased risks of kidney failure. You can see that even introducing seemingly healthy substances like resveratrol into medical practice can be challenging.
So let’s get into exercise mimetics more deeply. To understand SLU-PP-332, we need to take a look at another investigational compound. Unlike resveratrol which targeted SIRT1, this one activates the estrogen-related receptor gamma.
This is one of three siblings of the ERR family, expressed in tissues with high energy demands. These nuclear receptors have received considerable attention for their potential value in treating metabolic diseases. As a side note, nuclear receptors are proteins chilling in the cytosol or nucleus. They can sense specific ligand molecules and in turn, regulate expression of specific genes.
As their name suggests, ERRs are structurally related to estrogen receptors (ERs). These nuclear receptors utilize estrogens as ligands and contribute to breast and other cancer types. A key ER-drug is Nolvadex – more famously used by bodybuilders to manage their gynecomastia. Back to ERRs, which despite their resemblance work via different mechanisms than ERs.
The company GSK developed the first small molecule ERR agonists already in the early 2000s. Remember for later that this hydrazone-based agent strongly activates beta and gamma, but not the alpha ERR isoform. ERR beta is a negligible player given it’s not present in skeletal muscle .
ERR gamma highly expressed in oxidative slow-twitch muscle tissues in the calves, with minimal expression in quadriceps which appear more white. Its powerful effects can be clearly seen if a spooky experiment is performed, creating transgenic mice that express ERR gamma more broadly. These super mice have deep red muscle bellies due to improved oxidative capacity, increased vascularization and bigger mitochondria. In an endurance exhaustion test, transgenic mice ran roughly 1500 meters instead of measly 600m by wildtype mice. This means without any specific training, ERR overexpression creates endurance monsters that can run more than twice as far.
We also need to look at ERR alpha, the receptor which was not significantly activated by the GSK compound. Like the related gamma isoform, it’s expressed in skeletal muscle and has similar functions.
We’ve just seen how transgenic mice expressing ERR gamma are endurance monsters. For ERR alpha, scientists also looked at the opposite model – so called knockout mice lacking this important nuclear receptor. These mice are able to live somewhat normally which means that this receptor type is not vital for life. However, if you look at the relative size of the heart and muscles compared to body weight, the knockout mice in blue have significantly lower muscle mass.
As you might expect, this means the mice have lower endurance capacity and reach exhaustion much faster. The realization here is that if lacking ERR alpha results in endurance weakness, we could be able to mimic endurance exercise by activating it with a drug.
The first question is, how do we find an ERR alpha drug? One way is to start with the GSK compound – but wait, didn’t we say this one only activated the beta and gamma ERRs? I’ll explain. First, you have to know that the only available X-ray structure for this molecules is with ERR gamma. This tells us with high certainty what the binding mode looks like – for example, in red we can see that the phenolic hydrogen is involved in a hydrogen bond with an aspartate residue of ERR gamma. This structure can guide the simulation of how the slightly different binding pocket of ERR alpha would bind to ‘4716. As we’ve said the binding is not that strong, but we can use it as a starting point for the design of more potent drugs.
The scientists behind this research identified a crucial phenylalanine at position 328, here in pink, which is present in ERR alpha but not gamma. By engineering interactions with this unique group, we could design a drug that selectively targets alpha over gamma.
This was achieved very easily by converting the iso-propyl benzene of ‘4716 into a naphthalene ring. As you can see from the new simulation, this extended aromatic system can undergo pi-pi stacking with then phenylalanine. This simple change increases affinity for ERR alpha by more than 50-fold. Let’s compare it again to ERR gamma. As the phenylalanine is not present, the interactions are weaker here and the agent is around 4-to-1 selective for the alpha receptor.
The chemistry behind this is so easy that it can be managed by even the clumsiest undergrad . The starting materials are simple and cheap – the only thing needed is cooking them up in toluene overnight. The highly nucleophilic hydrazide adds to the electrophilic aldehyde, creating an adduct. After a proton shift, the intermediate can eliminate water, forming the hydrazone linkage of the product. As it precipitates from the solution, it can be easily separated and subsequently recrystallized to give pure SLU-PP-332.
Exercise mimetic effects
By now you are eager to hear about its effects – is it really as impressive as the clickbaity title? Let’s start from micro and go to macro. For some of these, feel free to pause and take more time to digest the info.
Upon treatment of isolated myocytes or muscle cells, the researchers observed a doubling of the maximal mitochondrial respiration rate. Obviously, more oxygen means a higher energy production. Not only are the mitochondria more productive, but there are more of them!
There were also structural differences. Here you can see stained sections from quadriceps muscle. Notice the difference? There’s significantly more green color which corresponds to myosin protein in type IIa fibers which are fast, aerobic muscles. On the other hand, there are less red, type IIb fibers. These are muscles which act fast but use anaerobic metabolism, meaning they fatigue quickly. Interestingly, no difference was observed for slow aerobic type I muscle.
Knowing this, what do you expect regarding exercise performance? Well, mice treated for a few days with this compound showed superior endurance without any training, being able to run roughly 70% further than normal mice. Unfortunately, the experimental procedure for this assessment is less fun. You can tell because the wording says “mice were run”. If they subsequently didn’t react to electrical shocks, you know that they were legitimately exhausted.
There are additional investigations into the “why” behind this such as specific gene expression targets. I leave this topic for interested nerds to check out on their own.
One interesting finding was that extended dosing for 2 weeks led to difference in grip strength as well. Unfortunately, the authors don’t describe this in more detail. It looks like grip strength decreased over time for both the active and the control group. This might be because of accumulating fatigue or other things. It would be cool if the authors are correct – meaning if SLU-PP-332 could enhance some strength endurance, and not just pure aerobic performance.
We haven’t covered one major, obvious question yet. The molecule’s exercise mimetic effects are very intriguing, but does it have any impact on weight?
Well, in another recent study the team documented that mice treated with the drug used more energy while consuming the same amount of food! Numerically it looks like they even had less food.
Metabolically, ‘332 triggered a shift towards fat burning. Fatty acid oxidation increased by roughly 25%, while the use of carbs decreased reciprocally. Again, more details can be found in the paper.
The magnitude of weight loss depends on the starting point. Normal mice with healthy diet and weight did not lose weight. On the other hand, obese mice eating an unhealthy high-fat diet saw 12% weight loss after one month. The researchers also looked at a genetic model. These mice don’t produce the key metabolic hormone leptin, leading to excessive hunger and food intake. Similar to the diet-induced obese mice, these chunky fellas also dropped significant weight.
We’ve noted that providing more than just weight loss is the next frontier. SLU-PP-332 might be one step in that direction. More pre-clinical work is required to understand its long-term effects. Maybe, related, optimized molecules could be even more potent. Researchers have not seen any safety signals but tolerability, administration and translation will be key to elucidate prior to first-in-human trials. These drugs will (likely) not be launched earlier than the end of the 2030s but with the excitement around obesity, it’s definitely going to remain an interesting space.
References on exercise mimetics
- PGC-1α, Inflammation, and Oxidative Stress: An Integrative View in Metabolism | Oxidative Medicine and Cellular Longevity, 2020, 1452696
- Caloric restriction and exercise “mimetics’’: Ready for prime time? | Pharmacological Research 2016, 103, 158
- Resveratrol improves mitochondrial function and protects against metabolic disease by activating SIRT1 and PGC-1alpha | Cell 2006, 127, 1109
- Distribution and Effects of Estrogen Receptors in Prostate Cancer: Associated Molecular Mechanisms | Frontiers in Endocrinology 2022, 12, 811578
- Identification and structure-activity relationship of phenolic acyl hydrazones as selective agonists for the estrogen-related orphan nuclear receptors ERRbeta and ERRgamma | J Med Chem 2005, 48, 3107
- Exercise and PGC-1 alpha-Independent Synchronization of Type I Muscle Metabolism and Vasculature by ERR gamma | Cell Metabolism 2011, 13, 283
- Estrogen-related receptor-α coordinates transcriptional programs essential for exercise tolerance and muscle fitness | Mol Endocrinol 2014, 28, 2060
- Synthetic ERRα/β/γ Agonist Induces an ERRα-Dependent Acute Aerobic Exercise Response and Enhances Exercise Capacity | ACS Chem. Biol. 2023, 18, 756
- A Synthetic ERR Agonist Alleviates Metabolic Syndrome | J. Pharmacol. Exp. Ther. 2023, 001733