At first there will be wash-out phase I with daily placebo administration for all participants. All patients will receive blood- and 24h-urine screenings in week 0 and 6 to get baseline data and also a low-dose native CT of the urinary tract in week 6. In preparation of the HCT phase all participants will daily receive an oral placebo.

In week 7 the HCT phase will start and all patients will receive an oral therapy with 50mg of HCT daily; the treatment response will be evaluated by blood- and urine test after 4 weeks of therapy (week 10).

After that, in washout phase II, the same test as before will be performed in week 0 and 6 (weeks 15 and 20).

At the end another therapy phase with an oral administration of 10mg dapagliflozin daily will be performed; the treatment response will be evaluated by blood- and urine test after 4 weeks of therapy (week 25).

Finally, the same tests as before will be performed in wash-out phase III in week 0 and 6 (weeks 29 and 34).

10mg Dapagliflozin daily for 8 weeks

As the primary endpoint the investigators defined the change of the urine oxalate excretion assessed by 24 h hour urine by 25% after 8 weeks of SGLT-2 inhibitor therapy compared to the baseline value that was collected in the washout phase before the treatment period. (Δ week0 and week 8 in mmol/L)

Reduction of the urine calcium excretion assessed by 24 h hour urine by 25% after 8 weeks of SGLT-2 inhibitor therapy compared to the baseline value that was collected in the washout phase before the treatment period. (Δ week0 and week 8 in mmol/L)