[Current Status and Regulatory Issues of Companion Diagnostics in Japan]

Companion diagnostics(CDx)are in vitro diagnostic products that are used to predict the efficacy and adverse effects of therapeutic drugs prior to administration, and are co-developed and co-approved with the therapeutic drugs in principle. In Japan, 40 CDx products have been approved by January 2024, and 39 products are used to determine if therapeutic drugs are applicable for cancer treatment. In the CDx products for cancer treatment, PCR, immunohistochemistry, or in situ hybridization is used to clarify the mutations(point mutations, insertions/deletions, fusions, etc.)in cancer-related genes or the expression levels of cancer-related molecules in the cancer tissues. The results of the analysis determine whether a particular therapeutic drug could be used or not for the treatment of the corresponding patient. Recently, several next-generation sequencing(NGS)-based CDx products have been approved and utilized for cancer treatment. The rise of NGS-based diagnostics has made it possible to comprehensively analyze mutations in many cancer-related genes in a single test and to determine whether each of several therapeutic drugs is applicable to the patient at once. On the other hand, with the increase in the number of CDx products, several regulatory issues have arisen, including an issue related to the co-development of CDx and a therapeutic drug and an issue related to the interchangeable use of CDx products that detect the same mutations of the cancer-related genes. The revision of CDx-related guidance is being considered in Japan and overseas in response to this situation.