Letter comments on: Drug–drug interactions between palbociclib and proton pump inhibitors may significantly affect clinical outcome of patients with metastatic breast cancer

I want to congratulate Del Re and their colleagues for their article1Del Re M. Omarini C. Diodati L. et al.Drug-drug interactions between palbociclib and proton pump inhibitors may significantly affect clinical outcome of metastatic breast cancer patients.ESMO Open. 2021; 6: 100231Google Scholar in which they investigated the effect of coadministration of proton pump inhibitors (PPIs) and palbociclib on progression-free survival (PFS) in patients with metastatic breast cancer (mBC). They reported that concomitant use of PPIs in patients with mBC treated with palbociclib has a detrimental effect on PFS. A total of 112 patients were enrolled in the study; 56 belonged to the ‘no concomitant PPIs’ group and 56 to the ‘concomitant PPIs’ group. However, the authors did not give detailed information about the number of metastatic sites in both groups. It would be expected that cases with higher metastatic sites might have a higher chance of decreased PFS. Second, worse outcome in PFS in patients with receiving palbociclib plus PPIs may be a class effect and not be generalizable to other oral cyclin-dependent kinase 4 and 6 inhibitors (abemaciclib or ribociclib). This issue merits further investigation. None declared.