The following tetrazole derivative of the formula (I) and a pharmaceutically acceptable salt thereof are known as a p-glycoprotein inhibitor, which has inhibitory activities on multidrug resistance in cancer cells (see KR Pat. No. 10-0557093):

wherein R1 to R11, m, n and X are the same as defined below.
P-glycoproteins are found in endothelial cells of the gastrointestinal tract, etc., and they are known to limit oral absorption of certain drugs. Some of the major anti-cancer agents such as paclitaxel, docetaxel and the like cannot be absorbed by the body mostly, because of the action of P-glycoprotein, if they were administered orally (Schinkel et al., Cell, 77, 491-502, 1994). One of the critical problems in the anti-cancer therapy is the expression of resistance towards anti-cancer agents in cancer cells and, among them, the most critical problems are multi-drug resistance (MDR) caused by overexpression of P-glycoprotein. In general, MDR in cancer cells increases as the use of anti-cancer agent increases, and this is a causative factor which substantially lowers cancer survival rates.
Accordingly, the P-glycoprotein inhibitor comprising a tetrazole derivative of the formula (I) can inhibit the action of the P-glycoprotein, thereby allowing oral administration of certain drugs and, thus, it is expected to be effective against the MDR in cancer cells that is induced by overexpression of P-glycoprotein.
Nevertheless, a tetrazole derivative and a pharmaceutically acceptable salt thereof have very low solubilities, and thus it is difficult to expect good in vivo absorption rate. Therefore, there is a need for improving the solubility and in vivo absorption rate of the aforementioned drug.