Objective To evaluate the release of leucine-enkephalin (L-EK) in the cerebrospinal fluid induced by intrathecal human embryonic kidney (HEK293) cells modified with human preproenkephalin (hPPE)gene and the analgesic efficacy of L-EK for bone cancer pain.Methods Forty CIBP female Sprague-Dawley rats were randomized into transplantation (CIBP+ hPPE/HEK293,n =20) and control (CIBP + HEK293,n =20)groups using a random number table.At 1 day before inoculation of cancer cells (T1,baseline) and 8,15,21,25,32 and 35 days after inoculation (T2-7),thermal paw withdrawal latency (TWL) was measured,and the number of licking/biting the claw on the transplantated side and degree of hindlimb limping during free activities were recorded.After observation at T4,10 rats were chosen from each group and sacrificed and the cerebrospinal fluid of rats was collected in an ice bath for detection of hPPE expression using radioimmunoassay.Results Compared with control group,TWL was significantly prolonged,the concentration of L-EK in the cerebrospinal fluid was increased,and the number of licking/biting the claw on the transplantated side and degree of hindlimb limping during free activities were decreased at T4-7 in transplantation group.Conclusion Intrathecal HEK293 cells modified with hPPE gene can continuously secrete L-EK and mitigate bone cancer pain.