Antitumor effects of residual tumor after cryoablation: the combined effect of residual tumor and a protein-bound polysaccharide on multiple liver metastases in a murine model

Abstract

Cryoablation is a low-invasive surgical treatment for malignant tumors. It may induce an immunological response leading to the eradication of distant metastases or alternatively it might promote the growth of residual tumors. In this paper we confirm the occurrence of both phenomena and we describe the preventive effect of a protein-bound polysaccharide preparation. Metastatic liver tumors were produced in BALB/c mice by the intrasplenic inoculation of colon 26 cells and cryoablation was carried out using liquid nitrogen (−170 °C) applied by a contact method. The value of combining cryoablation with administration of the polysaccharide preparation in the prevention of growth of residual tumors was investigated. It was shown that the number of metastatic liver nodules and the size of the primary tumor at the site of inoculation in the spleen were significantly lower when the volume that was frozen was small. The production by splenocytes of the tumor necrosis factor TNF-α, interferon INF-γ, and the interleukins IL-4 and IL-10 increased significantly after freezing and thawing of the tumor tissue. The polysaccharide treatment significantly reduced the production of IL-4 and IL-10 following cryoablation; the production of TNF-α and INF-γ was slightly promoted; the natural killer and cytotoxic T-cell activities of splenocytes were slightly enhanced. It was concluded that the polysaccharide preparation was beneficial by suppressing IL-4 and IL-10 production and might inhibit the growth of residual tumor that is sometimes induced by large-volume cryoablation.

Introduction

The treatment of malignant tumors by cryoablation has attracted attention because of its low invasiveness and relatively low cost. Considerable therapeutic effects on metastatic liver tumors [2], [16] and prostatic cancer [6], [15] have been reported. Cryoablation may not only destroy tumor directly by freezing and thawing, but may also induce a specific anti-tumor effect by an immune mechanism that is stimulated by tumor antigens released by the cryonecrotic tissue [1], [18]. Thus, cryoablation may be particularly promising in the treatment of advanced cancer with distant metastasis. However, it was observed in Japan from the late 1970s that cryosurgical treatment of tumors could have the negative effect of enhancing the growth of residual tumors and the method became less popular [5], [21]. This phenomenon was named high-zone tolerance.

We considered the possibility that the tumor enhancement and reduced survival might be induced by sensitization caused by a large amount of frozen and thawed tissue. We studied a metastatic liver tumor model in mice that was produced by the intrasplenic inoculation of colon 26 cells into BALB/c mice and we reported the preventative effects of biological response modifiers such as the protein-bound polysaccharide preparation we have called Krestin [19]. In the present study, we investigated the anti-tumor effect of cryoablation by a contact method on metastatic liver tumors and we discuss the mechanism underlying tumor enhancement and the preventative effects of Krestin.