Department of Ophthalmology and Visual Sciences
University of Michigan
Ann Arbor, Michigan

BASIC RESEARCH PROJECT

Developing a novel treatment to prevent vision loss due to recurrent retinal toxoplasmosis

Research Interests

Toxoplasma gondii (Tg) relies critically on a protease called Cathepsin L (TgCPL) to sustain chronic infection in humans. Chronic Tg infection can result in numerous human diseases, including ocular toxoplasmosis. Dr. Weh previously has shown that pharmacological inhibition of this enzyme reduces Tg burden in chronically infected animals. He proposes to test novel TgCPL chemical inhibitors for the treatment of ocular toxoplasmosis.

Dr. Weh hypothesizes that inhibition of the Tg Cathepsin L protease (TgCPL) will abolish chronic infection and prevent the development of ocular disease. To test this hypothesis, Dr. Weh will identify promising lead inhibitor compounds by determining retinal toxicity and efficacy in killing parasites in vivo. As a critical step toward the long-term goal of a safe, effective, and clinically available treatment for chronic Tg infection, the objective of this project is to define the role of selectivity for TgCPL over HsCPL along with the duration that injected compounds persist within the eye.

Plans for 2026

The project’s successful collaboration with Dr. Doggett has spurred continued broadening the scope of possible therapies by establishing another collaboration with Dr. L. David Sibley, affiliated with Washington University in St. Louis, who has developed anti-Tg compounds that can kill chronic Tg in mice. The Weh lab will test his compounds in our model of ocular toxoplasmosis to generate key pre-clinical data which will support further development of anti-Tg therapies to prevent recurrent ocular toxoplasmosis.

Towards the long-term goal of eliminating ocular toxoplasmosis, Dr. Weh and Dr. L. will test the efficacy of two classes of drugs targeting the TgCDPK1 and TgPheRS proteins in our pre-clinical mouse model of ocular toxoplasmosis. The working hypothesis is that pharmacological inhibition of TgCDPK1 and TgPheRS will reduce parasite survival in the lab’s animal model of ocular toxoplasmosis and thereby prevent recurrence of ocular disease. To test this hypothesis, the Weh lab will utilize their novel animal-based assay to assess loss of parasite viability following direct administration of compounds to the eye. This proposal aims to expand the treatment landscape for ocular toxoplasmosis to better identify effective therapies. The objective is to test the efficacy of direct delivery of TgCDPK1 and/or TgPheRS inhibitors to the eye to eliminate retinal Tg infection without causing systemic toxicity.

Specific Aims:
Aim 1. Determine the safety of TgCDPK1 and TgPheRS in the retina and dose
needed to attain tissue exposure levels sufficient to kill Tg parasites. At the completion of this aim, the optimal dosing of TgCDPK1 or TgPheRS inhibitors required to effectively kill Tg parasites will have been found.

Aim 2. Determine the extent to which TgCDPK1 and TgPheRS inhibitor effectively
kill Tg bradyzoites in the retina of our mouse model of ocular toxoplasmosis. At the conclusion of this aim, pre-clinical data for further development of therapies to prevent recurrent ocular toxoplasmosis will have been found.

Progress in 2025

Dr. Weh developed a model to readily quantify the effectiveness of anti-Tg compounds. With this new method his research shows up to 99.99% decrease in parasite survival following treatment. With additional funding secured in the year, the lab formed a new collaboration with Dr. J. Stone Doggett, affiliated with Oregon Health Science University, to expand the landscape of possible treatments to prevent ocular toxoplasmosis. This collaboration showed that these new inhibitors are non-toxic to the retina and easily reach retinal tissue to target Tg.

Progress in 2024

During 2024, the Weh laboratory tested two additional candidate compounds for retinal toxicity.  They did not find any evidence of toxicity to retinal structure or function, confirming previous data that TgCPL inhibitors are non-toxic to the retina. Pharmacokinetic experiments found TgCPL inhibitors present in the retina 7 days following a single IVT injection with TJB-3-64 at a therapeutic concentration. the team tested the efficacy of TJB-3-64 following two intravitreal doses, however no significant decrease in parasite burden was found. They then injected TJB-3-64 at 100x the previous dose into the eye, finding no evidence of retinal toxicity, but the pharmacokinetic analysis showed extremely rapid clearance, suggestingTJB-3 64 may be crossing the blood-retina barrier. Systemic administration confirmed the ability for this compound to cross the blood-retina and blood-brain barriers.

During 2024, the lab focused more on compounds that have higher selectivity for the parasite enzyme over the human form of the enzyme. The data from Triazine-3 suggest that this selectivity may be the key to specifically targeting and killing toxoplasma parasites. Dr. Weh also investigated the pharmacokinetic properties of these compounds to identify candidates that remain within the eye for longer periods of time. Additionally, by using a renewed strain of parasite, the lab team is able to induce a higher intraocular parasite burden, making the detection of even a small decrease in parasites following drug treatment more effective.

Progress in 2023

Dr. Weh’s laboratory team was able to test for toxicity of an additional candidate compound (Triazine-3). This compound was also found to have no significant effect on retinal health or function following a single intravitreal dose. They tested the efficacy of Clindamycin and LHVS following a single intravitreal dose; however, could not detect any significant decrease in parasite burden. A pharmacokinetic study found that LHVS is rapidly cleared from the eye and is undetectable by 24 hours after injection.  The team performed a new efficacy study (LHVS and Trizaine-3) using 2 IVT doses one week apart. Again, no significant decrease in parasite numbers was found; however, there is trend towards a decrease in parasite numbers with Triazine-3, suggesting that selectivity for the parasite enzyme may be important.