Recent applications of chemosensitivity tests for colorectal cancer treatment

Table 1 Overview of the cell culture-based chemosensitivity tests

Name	Studied tumor type	Description
MTT[17,18]	Breast and stomach	The MTT assay measures mitochondrial activity and is most often used to detect loss of cell survival/cell viability in response to a drug or toxin. Tumor cell suspensions are cultured with various chemotherapy agents for 3-4 d and then exposed to the MTT reagent; because it reduces intracellularly to a blue dye, the intensity of uptake yields an estimate of the number of viable cells to determine drug sensitivity
HDRA[5,21,22]	Stomach, breast, ovary, and colon	The HDRA uses cancer tissue fragments and three-dimensional cell culture, in which intercellular contacts and interactions with stromal cells are maintained. Tumor specimens are cut into 1-mm3 pieces and put on a gelatin sponge infiltrated with culture medium containing a test drug. After incubation for 3-7 d, cell viability is assessed using the MTT assay
ATP[6,11-14]	Ovary, breast, stomach, and colon	The quantification of intracellular concentrations of ATP as a measure of cell survival has gained wide acceptance for the evaluation of the medium and long-term cytotoxic effects of drugs (2-3 d). The assay is based on the bioluminescent detection of cellular ATP and is extremely sensitive, allowing the measurement of ATP levels in a single adherent or non-adherent mammalian cell
EDRA[26,31]	Ovary, breast, lung, and colon	After 3-5 d of culture, tumor cells obtained from fresh biopsy specimens are labeled with tritiated thymidine. The level of uptake is tracked after exposure to chemotherapy drug concentrations that approximate the peak level achieved clinically. Extreme resistance is identified when thymidine incorporation is inhibited in the presence of the drug by less than one standard deviation of the median cell inhibition measured for several hundred reference tumor samples

MTT: Methyl thiazolyl-diphenyl-tetrazolium bromide; HDRA: Histoculture drug response assay; ATP: Adenosine triphosphate bioluminescence; EDRA: Extreme drug resistance assay.

Table 2 Protein and gene-based chemosensitivity tests in colorectal cancer

Marker	Target chemotherapy drug	Function	Change	Consequence
TS[34,35]	5-FU	Essential enzyme for DNA synthesis	TS expression ↓	1Chemotherapy response ↑
DPD[33,35]	5-FU	Degradation of 5-FU	DPD expression ↓	1Chemotherapy response ↑
TP[39]	5-FU	Activation of 5-FU (from 5’-DFUR to 5-FU)	Stromal TP expression ↑	1Chemotherapy response ↑
UGT1A1[49]	Irinotecan	Degradation of the active metabolite of irinotecan (SN-38)	Polymorphism of UGT1A (UGT1A1*28)	Irinotecan toxicity↑
ERCC1[54]	Oxaliplatin	Excision nuclease that repairs platinum-induced DNA adducts	ERCC1 expression ↓	1Chemotherapy response ↑
KRAS[65-69]	Anti-EGFR	Proto-oncogene in the EGFR signaling pathway	Mutation of the KRAS gene	Chemotherapy response↓
NRAS[72]	Anti-EGFR	Proto-oncogene in the EGFR signaling pathway	Mutation of the NRAS gene	Chemotherapy response↓
BRAF[74-77]	Anti-EGFR	Signaling gene acting downstream of KRAS	Mutation of the BRAF gene (V600E)	Chemotherapy response↓

¹Chemotherapy responses of these markers are generally inconsistent without strong evidences. TS: Thymidylate synthase; 5-FU: 5-fluoropyrimidine; DPD: Dihydropyrimidine dehydrogenase; TP: Thymidine phosphorylase; 5’-DFUR: 5’-Deoxy-5-fluorouridine; UGT1A1: Uridine diphosphate glucuronosyltransferase 1A1; ERCC1: Excision repair cross-complementation group 1; anti-EGFR: Anti-epidermal growth factor receptor (cetuximab or panitumumab).