#512 Two are better than one: a new combination treatment for cystinosis
Authors: Cheung Pang Yuk , Wu Chia-Hua , Davidson Alan , Hollywood Jennifer 
Published: May 2024

Abstract

Background and Aims
Nephropathic cystinosis is a rare, lysosomal storage disorder resulting from mutations in the CTNS gene, encoding a cystine transporter. Disruption of this protein leads to the accumulation of cystine in cells and the formation of crystals that damage tissues and organs. Patients usually present around 6-months of age with failure to thrive, excessive thirst (polydipsia) and urination (polyuria) and Fanconi syndrome (excessive loss of important nutrients in the urine). The kidneys are seriously affected due to proximal tubule dysfunction and if left untreated this can lead to kidney failure by the age of 10. Patients receive the drug-based therapy cysteamine from the time of diagnosis. However, even with prolonged treatment, individuals may still advance to kidney failure, necessitating transplantation. As such, there is an urgent need for alternative treatments. In previous work using stem cell and kidney organoid models, we discovered that a drug combination of cysteamine and the mTOR inhibitor, Everolimus, can correct the cystinotic cell phenotype. To evaluate the therapeutic potential of this therapy in vivo, we generated a cystinotic rat model that faithfully recapitulates the human disease phenotype within 3–6 months as seen by: failure to gain weight, polydipsia and polyuria, cystine accumulation, Fanconi syndrome and kidney dysfunction. Using these Ctns knockout (KO) rats we performed pre-clinical testing to evaluate the therapeutic potential of Cysteamine/Everolimus combination treatment.

Method
Six-week-old male Ctns KO rats (n = 6) were dosed with either vehicle, Cysteamine-only, Everolimus-only or combination (Cysteamine and Everolimus) delivered via jelly pills for 6 months. Body weights were recorded weekly and blood and urine collected monthly for in-depth chemical analysis of Fanconi syndrome markers. At the end of the study, tissues were harvested for cystine measurements, immunohistochemistry and histology.

Results
Cysteamine monotreatment was efficacious in ameliorating the disease phenotype while Everolimus alone showed mixed results. However, combination treatment resulted in a superior reduction in tissue cystine levels, polydipsia, polyuria and a superior improvement in Fanconi syndrome markers, gross kidney morphology and histology compared to monotreatments. Specifically, in-depth urine analysis at 6 months showed significantly reduced levels of albumin, total protein and glucose in combination-treated animals compared to cysteamine monotreatment. Furthermore, the kidneys of combination-treated animals showed a greater amount of undamaged proximal renal tubules when compared to vehicle, cysteamine and Everolimus-treated kidneys along with a greater preservation of Lrp2/Megalin.

Conclusion
Overall the combination treatment resulted in greater preservation and protection of renal structures compared to monotreatments. These results demonstrate the potential of a Cysteamine/Everolimus dual therapy to improve the treatment of cystinosis.