Role of circulating tumor cell spheroids in drug resistance

Abstract

Cancer cell spheroids are used for drug screening as these three-dimensional (3D) assemblies recapitulate tumors more realistic than the widely employed 2D in vitro cultures. Limited drug diffusion and gradients of oxygen and nutrients in spheroids represent avascular tumor regions containing quiescent and hypoxic tumor cells with high drug resistance. Circulating tumor cells (CTCs) effect metastatic spread and are present in high numbers in malignant diseases such as small-cell lung cancer (SCLC) and in other cancer patients with high tumor load. CTCs are heterogeneous and only a small fraction of these cells survive in the circulation and cause distal lesions. CTCs may circulate as single cells but small CTC clusters or CTC spheroids have been detected in cancer patients and demonstrated to possess increased metastatic potential. At our lab we have obtained 9 permanent SCLC CTC cell lines (BHGc7, BHGc10, BHGc16, BHGc26, BHGc27, BHGc50, BHGc59, BHGc71, and UHGc5) of distinct patients exhibiting similar characteristics and spontaneous formation of large spheroids, termed tumorospheres. These aggregates were shown to exhibit high drug resistance compared to the corresponding single cell suspensions. The increased metastatic capability of small circulating tumor clusters/spheroids may be explained by their role as putative precursors of tumorospheres eventually trapped in capillaries. Limited drug penetration and the presence of hypoxic/quiescent cells can readily account for the global drug resistance of advanced SCLC which has resulted in clinical failure of a wide range of chemotherapeutics and low survival. Furthermore, we have detected such tumorospheres in non-small-cell lung cancer (NSCLC) patients progressing under EGFR-directed tyrosine kinase inhibitor therapy which had undergone NSCLC-SCLC transformation.