Treating Parkinson’s and other neurological conditions has been challenging due to a lack of tools capable of navigating the complexity of neural circuits. New precision tools like DART.2 help make those therapeutic aspirations a reality, one tethered drug at a time.
The brain is one of our most complex organs, full of neurons that are constantly communicating with one another at places called synapses. Synapses both release molecules called ligands and express cell surface receptors that the ligands bind to, prompting the cells to undergo various processes within. The kinds of ligands and receptors that are important to disease are often released by and found on a variety of cell types. It is crucial, therefore that when we use a drug that targets a certain receptor, we also make sure that they only interact with receptors on the desired cell type.
To achieve this, in 2018 a team led by Duke professor Mike Tadross introduced “Drugs Acutely Restricted to Tethering” or “DART,” a drug delivery system that allows researchers to administer drugs to specific neuronal cell types. In June of this year, the Tadross lab unveiled DART.2. Pairing the cell-type specificity provided by DART.2 with the cellular receptor-specificity already provided by a given drug is essential to treating diseases like Parkinson’s without severe off-target effects, something researchers have been unable to do until tools like DART entered the scene.
Think of it like cutting the water supply to an apartment that has had a pipe burst. You don’t want to cut the water to the entire building, just the flooding apartment, because the other tenants still need water. This newest version of DART increases the ability of researchers to flip the right switches.
With increased cell type specificity optimized for drugs targeting two different receptor types, enabling broader dosing techniques and opening the door for discoveries of unknown roles of well-known receptors, researchers have made DART.2 into an “even more subtle, refined, yet transformative drug delivery system, a marked improvement from more rudimentary options that operate more like sledgehammers,” said Brenda Shields, one of the lead scientists on the DART.2 project.
This is, in part, owing to the use of natural, or endogenous, receptor machinery in its design. Organisms are infected with a virus that prompts only certain cell types to express HaloTag, a protein that sits on the surface of the cells of choice. A HaloTag ligand, or small molecule that binds specifically to the HaloTag surface protein, is tethered to a drug of choice. This allows the tethered drug to be selectively recruited to the cells that have the HaloTag protein, bringing the drug into closer proximity to its intended cellular receptor, discouraging it from binding to unintended receptors, and reducing the amount of drug needed for efficacy.
DART.2 is not changing receptors that are already present, nor is it affecting the signaling cascades activated by engagement with the receptors. Cell-specificity of drug delivery was improved in DART.2 by decreasing the time and drug concentration needed to achieve intended effects – it is 100 times more precise than the previous system, with desired effects achieved in just 15 minutes.
Using the previous version of DART, researchers tried delivering a tethered version of the drug gabazine to GABA receptors (neural receptors associated with inhibitory neurotransmission) on a select group of neurons – gabazine blocks GABA from binding to GABA receptors and subsequently increases neural activity. Unfortunately, DART did not achieve high enough cell-specificity and gabazine bound to enough off-target receptors to trigger epileptic responses in mouse models. DART.2, however, is capable of delivering gabazine without these effects. The original version of DART was only optimized to work with drugs targeting excitatory (AMPA receptors) neurotransmission. The ability of the current version to work with inhibitory (GABA) and excitatory (AMPA) neurotransmission makes this system useful for “bi-directional” modifications, greatly increasing its utility.
Interestingly, while testing the effects of the drug gabazine on GABA receptors in ventral tegmental area dopamine neurons, they found that GABA receptors on these cells actually suppress locomotion, opposite to findings in other studies that more broadly focused on GABA receptors in multiple cell types. This highlights the need for tools like DART.2 that allow us to understand diverse receptor/ligand dynamics on a cell-by-cell basis to gain more nuanced approaches to understanding and treating disease.
To visualize dispersion and binding of tethered drugs to HaloTag proteins versus off-target receptors, Tadross’s team developed a way of seeing where the tethered drugs accumulated by introducing a small percentage of HaloTag ligands bound to a fluorescent reporter rather than a drug into the pool of tethered drugs. This visualization further confirmed a significant increase in cell-type specificity and a decrease in off-target effects of DART.2.
With previous levels of cell specificity, local delivery of the tethered drugs via cannula insertion at the brain region of interest was necessary to ensure drugs made it to the right targets. With increased specificity and a new visualization method for seeing where DART.2 drugs bind, researchers were able to assess whether brain wide dosing would be possible, decreasing deleterious effects of pumping high concentrations of drug into a certain area. Excitingly, they found that broad administration of tethered drugs across large areas of the brain did not significantly increase binding at receptors on cells not expressing the HaloTag protein. Drug delivery in the brain is notoriously difficult, so having the flexibility to administer a drug from an easier delivery point without reducing binding at target sites translates to a greater chance of therapeutic success.
For those unfamiliar with the process of drug and therapeutic development, the improvements presented in DART.2 represent a realistic look into the measure of scientific progress. Originally used to treat Parkinson’s mouse models, our conception of DART.2’s therapeutic relevance to other conditions is continually expanding. Shields shared that adaptions for conditions such as anxiety and depression may not be far off, just to name a few. DART.2 also makes it possible to use drugs like opioids in new ways. While helpful for pain management, the addictive potential of opioids sometimes renders them more harmful than helpful. Utilizing DART.2, opioids could be administered more specifically to cells that benefit from the drug, potentially reducing interactions with cell types involved in addiction development. Additionally, researchers are beginning to couple DART.2 with other tools on the market that can enhance its therapeutic promise and applicability (and vice versa). Each improvement of DART brings us closer to the reality of treating conditions we once deemed hopeless.
