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Background: Primary hyperoxaluria (PH) is a rare, genetic disorder which involves the overproduction of endogenous oxalate, leading to hyperoxaluria, recurrent urolithiasis and/or progressive nephrocalcinosis and eventually resulting in kidney failure and systemic oxalosis. The aim of this trial was to investigate whether treatment involving an oxalate-metabolising bacterium ( Oxalobacter formigenes) could reduce urinary oxalate excretion in PH patients. Methods: The efficacy and safety of O. formigenes (Oxabact® OC5; OxThera AB, Stockholm, Sweden) was evaluated in a randomised, placebo-controlled, double-blind study for 8 weeks. The primary objective was reduction in urinary oxalate excretion (Uox). Secondary objectives included faecal O. formigenes count and decrease in plasma oxalate concentration (Pox). Results: Twenty-eight patients randomised 1:1 to the treatment group (OC5) or the placebo group completed the study. After 8 weeks of treatment, there was no significant difference in the change in Uox (mmol/24 h/1.73 m) between the groups (OC5: +0.042, placebo: −0.140). Post-hoc analysis showed a statistically significant increase in Uox per urinary creatinine excretion in the OC5 group (OC5: +5.41, placebo: −15.96; p = 0.030). Change in Pox from baseline was not significantly different between groups ( p = 0.438). The O. formigenes cell count was significantly increased in OC5-treated patients ( p < 0.001) versus placebo. The treatment response to O. formigenes was related to individual stage of kidney deterioration, and Pox was directly correlated to kidney function, even for early-stage patients (chronic kidney disease stage 1). No safety issues were observed. Conclusions: Treatment with OC5 did not significantly reduce Uox or Pox over 8 weeks of treatment. The treatment was well tolerated and successfully delivered to the gastrointestinal tract. [ABSTRACT FROM AUTHOR]