The in vivo use of dithiothreitol in cystinosis

Two male patients with late stage (uremic) infantile nephropathic cystinosis (INC) (Table 1) were treated by mouth with the reducing agent dithiothreitol (DTT), at doses not exceeding 25 mg-kg-1 body weight three times per day. Three sequential periods of observation were obtained in both patients: on thiol (8.5 months); off thiol (8-9 months); on thiol again (7 months or longer). Other than nausea and vomiting at the maximum dose range, no apparent toxicity was observed. One subject died in uremia in the 24th month of the study. The half-cystine concentration in peripheral blood leukocytes decreased during both treatment periods in each patient from initial pretreatment levels in excess of 8 nmol-mg-1 protein (normal less than 0.1 nmol-mg-1) to 10-20% of initial values (Table 2 and Fig. 1, A and B). Reduction in total number of blood leukocytes or in the neutrophil fraction, where cystine storage occurs selectively in cystinosis, did not occur (Table 3) as a possible explanation for these findings; nor did storage of samples, a possible artifact, influence the cystine content of cystinotic cells (Fig. 2). Multiple site rectal mucosa biopsy clearly revealed cystine storage but serial biopsies did not reflect a positive DTT response when compared with the leukocyte assay (Table 4). High intersample variation in cystine content, even between samples taken at one time, prevented measurement of a treatment response. DTT had no apparent detrimental effect on the concentration of representative proteins, including hemoglobin (Table 3), serum insulin, and serum immunoglobulin during the treatment trials. Renal function (glomerular and tubular) was severely depressed and did not improve during the period of observation in either patient (Table 2; Fig. 3, A and B). Postmortem tissues from one patient revealed 10-40-fold excess cystine accumulation in kidney cortex and liver (Table 5). However, these levels of accumulation are at the lower range of or even below published values for cystine in cystinotic kidney and liver. Whereas chemical methods are not reliable for detecting and measuring DTT in biologic fluids, preliminary evidence indicates that a silylated derivative of oxidized DTT can be detected in the urine of patients receiving DTT by mouth (Fig. 4). This finding suggests that the thiol is absorbed and excreted.