Scientists at Fred Hutchinson Cancer Research Center have harnessed a tiny protein found in scorpion venom as a targeted delivery system for carrying steroids directly to the cartilage of rats with rheumatoid arthritis (RA), potentially avoiding the severe toxicity and risk of infection associated with current systemic systemic approaches. The team’s studies showed that the steroid-coupled venom miniprotein accumulated specifically in joint cartilage, and reversed inflammation in treated animals. “For people with multi-joint arthritis, the side effects of controlling the disease can be as bad or worse than the disease itself,” said the project’s senior scientist Jim Olson, MD, PhD, a member of the clinical research division. “Steroids like to go everywhere in the body except where they’re needed most. This is a strategy to improve arthritis relief with minimal systemic side effects.”

Olson and colleagues reported their findings in Science Translational Medicine, in a paper titled, “A potent peptide-steroid conjugate accumulates in cartilage and reverses arthritis without evidence of corticosteroid exposure.”

More than two million people in the United States live with chronic pain and associated limitations resulting from diseases such as RA that cause multi-joint inflammation, the authors wrote. Existing therapies can help, but they can also cause severe toxicities. “Current therapies are mechanistically capable of reducing inflammation and concomitant cartilage degradation, but to achieve these benefits, they also cause severe toxicities in other tissues and increase the risk of infection in patients … Although chronic systemic use of potent steroids is no longer the clinical standard of care because of pernicious cumulative toxicities, the fact remains that steroids are often effective.”

What is needed are new approaches to delivering therapeutic agents directly to joints and minimize systemic exposure, which would improve the therapeutic window of drugs that are known to be effective but which can’t be used long term because of on-target toxicity in healthy tissues. “Development of a safe and effective targeting agent that can deliver a variety of systemically administered therapeutics to all joints throughout the body would be groundbreaking for the fields of orthopedics and rheumatology,” the investigators noted.

Olson and his team have for years studied tiny proteins, or optimized peptides—which they call “optides”—derived from organisms including scorpions, snakes, and flowers such as violets and sunflowers. CDPs are a class of such miniproteins that are typically about 20 to 60 amino acids in length, the team wrote. “They are frequently found in the venom of spiders, snakes, and scorpions as biodefense mechanisms, but they are also made by a wide variety of other species, including plants, fungi, bacteria, marine mollusks, and insects.” As Olsen explained, “My thought was that these peptides that are in venoms or toxins might have really unique biodistribution in human bodies. If something is using them for predation, they need to get to certain places rapidly.”

More than a decade ago, Olson discovered a CDP in the death stalker scorpion that specifically binds to cancer cells. He co-founded a company, Blaze Bioscience, in 2010 to develop an experimental dye called Tumor Paint BLZ-100, which comprised a glowing version of the death stalker’s cancer-targeting protein. This is now being tested as a tool for surgeons to precisely illuminate hard-to-see brain tumors. Having developed the tumor-homing toxin-derived peptide, Olsen reasoned that there may be CDPs with other targeting properties. “On the basis of the development of the tumor-homing modified chlorotoxin CDP as a potential oncology surgical aid, we hypothesized that other CDPs may have therapeutic utility stemming from their native properties,” the investigators commented.

The cartilage-accumulating scorpion toxin was identified by Olsen and his team through research that involved screening dozens of peptides derived from scorpions and spiders. The original aim was to find molecules that could cross the blood-brain barrier. However, one of the CDPs, designated CDP-11R, appeared to accumulate and remained in cartilage, throwing up the possibility that it could be harnessed for treating arthritis. “It really shows the value of playing scientifically and just doing things for the pure joy of learning,” Olson said. “You never know where it’s going to take you. If we could relieve arthritis for millions of people with very few side effects, that’s a really good investment of our time.”

Olson worked for several years with colleagues at the Fred Hutchinson’s Molecular Design and Therapeutics core facility to develop a way to link CDP-11R to drugs. The team initially coupled the peptide with a fluorescent molecule, using a chemical linker, and confirmed that the conjugate accumulated in and was retained in human cartilage taken from postmortem donors. They then paired CDP-11R to the steroid dexamethasone (dex) using a chemical linker. However, tests with this conjugate in rats showed that small amounts of the drug leaked into the rats’ bloodstream, causing the same side effects that they were hoping to eliminate. “ … because of the potency and lack of metabolism to inactive metabolites, dex released from CDP-11R caused on-target pharmacodynamic responses that would be dose-limiting due to systemic steroid exposure when used in the long term care of human patients,” the scientists reported.

They then tested another conjugate, in which the miniprotein was linked with a different steroid, triamcinolone acetonide (TAA). “To demonstrate efficacy at the joint and reduced potential toxicity when using the CDP conjugate, we switched the active payload to triamcinolone acetonide (TAA),” they stated. “The higher clearance of TAA, compared to dex, may mitigate the effect of any systemic exposure of released drug,” they reasoned.

Encouragingly, experiments in the rat model of rheumatoid arthritis showed that the conjugate was as effective as dexamethasone at treating inflammation. But while the dexamethasone conjugate caused toxicities, TAA becomes inactive when it enters the bloodstream, so there were no detectable side effects. “ … the CDP-11R–TAA conjugate alleviated joint inflammation in the rat collagen–induced model of rheumatoid arthritis while avoiding toxicities that occurred with nontargeted steroid treatment at the same molar dose,” the team wrote. “This conjugate shows promise for clinical development and establishes proof of concept for multijoint targeting of disease-modifying therapeutic payloads.”

The team says their results support further development of CDP-steroid conjugates, but also pave the way for testing potential nonsteroid disease-modifying payloads that might help to reduce pain or even help to repair or restore cartilage. The team hopes to test this approach in patients through a clinical trial, Olson said. But first they’ll need to answer several more unknowns with further toxicology and other studies. “Although the experiments described here establish proof of principle that CDPs could be used to deliver therapeutic agents to diseased joints, further investigations specific to the payload and indications will be indicated in the course of candidate development.”

Emily Girard, PhD, a staff scientist in Olson’s lab and one of the study’s lead authors, stated, “It’s a pretty simple idea to take a miniprotein that naturally goes to cartilage and attach something to it so that you get targeted delivery of the drug, but it was challenging to accomplish. We had to learn and adapt the behavior of the miniprotein, the chemical linker, and the steroid payload to make a product that would go to cartilage, stay as long as we needed it to, release the drug at the right rate, and have a local but not systemic effect. There is more development to be done, but I hope that this work results in a therapeutic that will help a lot of people.”

Olson added that while this study involved steroids, it shows that these miniproteins could deliver other drugs into cartilage. “We think that steroids have important potential as a candidate for clinical development and we’re actively exploring other payloads that could be delivered to the joints,” he said. “The long-term goal is to deliver molecules that go beyond controlling arthritis to actually reversing it.”