Ultrasound Guided Combined Cryoablation and Microencapsulated 5-Fluorouracil Inhibits Growth of Human Prostate Tumors in Xenogenic Mouse Model Assessed by Luminescence Imaging

Modern approaches to minimally invasive ablative treatment of solid tumors involve the use of miniature instruments and combined treatments. These can be enhanced with ultrasound imaging that depicts tumor margins; facilitates guidance, delivery, and dosage of local chemotherapy; and can monitor the effectiveness of the treatment. This paper describes the advantages of ultrasound guided cryosurgery combined with local chemotherapy delivered in multilamellar, echogenic microcapsules of 5-FU (“μcaps”) using a xenograft tumor model. Genetically engineered bioluminescent human prostate tumor cells, DU-145Luc+, were implanted subcutaneously into athymic nude mice. Experiments were designed to mimic the situation where palliative cryoablation spares a portion of the tumor so that the combined effect of cryosurgery and focal injections of chemotherapeutic microcapsules could be evaluated. Eighteen (18) tumors were treated with percutaneous partial cryoablation or interstitial chemoablation, or a combination of both. A single F/T cycle was applied to tumor and micro-encapsulated chemotherapy is delivered at outer margin of frozen tumor in two opposite sites. Results show that the tumor and cryosurgical kill zone contours were seen with both the bio-luminescence assay (BLI) and ultrasonography (US). US can easily detect as little as 2 μl of echogenic μcaps, and monitor their lifetime in the tumor tissue. BLI was determinant in showing that minute amounts of microcapsule chemotherapy (38.7 ng of 5-FU/g tumor) dramatically inhibited tumor growth starting within two days after injection. The mean BLI emitted by control tumors was 5.6 times greater at Day 4 than the BLI measurements from tumors treated with 5-FU μcaps (p=0.036). By Day 7, BLI values from the control tumors were still 2.7 times greater than those treated with 5-FU μcaps (p<0.01). In tumors treated by partial cryoablation, the mean BLI of viable tumor cells was 20 times less at day 3 (p=0.05) and 46% less at day 7 than the non-treated tumors. The combined treatment produced a dramatic inhibition of tumor growth that lasted throughout the 7-day study. The BLI measured from viable tumor cells in non-treated tumors was 34 times greater at day 3 and more than 350 times greater at day 7 than those treated by combined cryoablation and 5-FU μcaps. The results demonstrated, for the first time, that a single moderate freeze of a human prostate tumor combined with bi-focal peripheral microcapsule chemotherapy (5-FU) has a better and longer inhibitory effect on tumor growth compared to the growth inhibition rendered by cryosurgery or local microcapsule chemo-therapy alone. This shows promise for a new, focal, combined ablative modality using US guided deposition of microencapsulated drug(s) and echogenic markers deposited in the hypothermic margin of tumors which could enhance the efficacy of cryoablation of prostate cancers.