Up to 30 patients with CFRD on highly effective CFTR modulator therapy who meet criteria and agree to participation in the study will be placed on metformin 500mg twice daily (low) and 1000mg twice daily (normal) in a randomized order (simple randomization). There will be a dose-escalation with each dosing regimen starting with 500mg twice daily for a week, followed by 500mg in the AM and 1000mg in the PM for another week and finally followed by 1000mg twice daily until the end of normal dose cycle (in those in the normal dose portion of the crossover trial). A matching placebo pill will be utilized so participants do not know which dosing regimen, low or normal, they are on during each 14-week period. Participants will continue each dosing regimen of metformin for 14 weeks with a washout period of 2 weeks between dose changes. To minimize risk of B12 deficiency, a known side effect of long-term metformin use, we will also provide a supplement of 1000 µg oral cyanocobalamin daily for the duration of the trial.
The purpose of this study is to assess the efficacy of metformin to improve airway ion channel function in those with CF-related diabetes (CFRD)