Rectal adenocarcinoma coexisting with incidentally found microscopic gastrointestinal stromal tumor: A case report

Inhibition of human UDP-glucuronosyltransferase enzyme by ripretinib: Implications for drug-drug interactions

Ripretinib, a tyrosine kinase inhibitor (TKI), is the first FDA approved fourth-line therapy for adults with advanced gastrointestinal stromal tumor (GIST). Studies have shown that several TKIs for treating GIST were potent inhibitors of human UDP- glucosyltransferase (UGTs) enzymes. However, whether ripretinib affects the activity of UGTs remains unclear. The aim of this study was to investigate the effects of ripretinib on major UGT isoforms, as well as to evaluate its potential drug-drug interactions (DDIs) risk caused by the inhibition of UGTs activities. The inhibitory effects and inhibition modes of ripretinib on UGTs were systematically evaluated using high-performance liquid chromatography (HPLC) and enzyme kinetic studies, respectively. Our data showed that ripretinib exhibited potent inhibition against UGT1A1, UGT1A3, UGT1A4, UGT1A7 and UGT1A8. Enzyme kinetic studies indicated that ripretinib was not only a competitive inhibitor of UGT1A1, UGT1A4 and UGT1A7, but also a noncompetitive inhibitor of UGT1A3, as well as a mixed inhibitor of UGT1A8. The prediction results of in vitro-in vivo extrapolation (IVIVE) demonstrated that ripretinib might bring the potential risk of DDIs when combined with substrates of UGT1A1, UGT1A3, UGT1A4, UGT1A7 or UGT1A8. Therefore, special attention should be paid when ripretinib is used in conjunction with other drugs metabolized by UGTs to avoid risk of DDIs in clinic.

Rectal gastrointestinal stromal tumor: an unusual presentation of an uncommon pathology (a case report)

Gastrointestinal stromal tumours (GIST) are a rare form of neoplasm. The stomach is the commonest location while gastrointestinal bleeding and pain are the usual presentations. Rectal GIST has been reported in literature as a rare occurrence. We report the rare case of a 37-year-old man who presented with large bowel obstruction and acute urinary retention arising from a rectal GIST. Radiological investigations showed features in keeping with intestinal obstruction. He had a divided colostomy and tumour debulking. Histology of tumour revealed a rectal GIST and immunohistochemical staining was positive for CD34 and CD117. Postoperatively Imatinib was commenced and patient did well. We report this case to highlight the unusual symptoms that may arise from a rare pathology like rectal GIST and the need to consider an alternative diagnosis-such as GIST, in a young adult presenting with large bowel obstruction in the absence of risk factors for bowel adenocarcinoma.