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. We have shown previously that pharmacological inhibition of this enzyme reduces Tg burden in chronically infected animals. We propose here 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 2025

To test his hypothesis further, Dr. Weh will measure the loss of Tg cyst viability after TgCPL inhibition and identify novel compounds that can cross the blood-retina and blood-brain barriers. 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 proposal is to develop a novel method for quantifying Tg cyst viability using reinfection models.

Dr. Weh has developed a novel method which he believes will more accurately and reliably detect a decrease in cyst viability after in vivo treatment. Parasites isolated from treated or untreated animals will be used to identify the minimum amount of sample needed to cause reinfection. At the same time, his progress last year showed that his current lead compound, TJB-3-64, can cross the blood-retina barrier. Dr. Weh’s lab will continue our medicinal chemistry efforts to optimize this class of compounds to enhance the ability for TgCPL inhibitors to enter into the retina and brain following systemic administration. This is an important step towards developing therapeutic compounds that can be easily, and safely, administered to patients suffering from ocular toxoplasmosis.

Specific Aims:
Aim 1. Develop novel methods for quantifying loss of cyst viability in vivo using reinfection models. At the completion of this aim we will have shown TgCPL inhibitors can kill Tg cysts in vivo.
Aim 2. Identify new lead candidate TgCPL inhibitors with prolonged tissue retention times after in vivo administration. At the completion of this aim we will have identified a new TgCPL inhibitor that reaches the retina at therapeutic concentrations after systemic administration.

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.

Specific Aims:
Aim 1. Assess safety and intraocular duration of novel TgCPL lead compounds when delivered directly to the retina. Dr. Weh will deliver TgCPL lead-compounds directly to the retina using intravitreal (IVT) injection and assess the overall toxicity and length of time these compounds remain in the retina.
Aim 2. Evaluate the efficacy of novel TgCPL inhibitors in a model of chronic ocular toxoplasmosis. Mice experiencing chronic Tg infection will be treated with highly selective TgCPL inhibitors via IVT dose to eliminate chronic stage parasites from the retina.

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.