2.1 The CHO/HGPRT assay detects forward mutations of the X-linked hypoxanthine-guanine phosphoribosyl transferase (hgprt) locus (coding for the enzyme, HGPRT) in Chinese hamster ovary (CHO) cells. Cells originally derived from Chinese hamster ovary tissue are exposed to a test article and, following an appropriate cell culture regimen, descendants of the original treated population are monitored for the loss of functional HGPRT, presumably due to mutations. Resistance to a purine analogue, 6-thioguanine (6TG) (or less desirably, 8-azaguanine (8AG)), is employed as the genetic marker. HGPRT catalyzes the conversion of the nontoxic 6TG to its toxic ribophosphorylated derivative. Loss of the enzyme or its activity therefore leads to cells resistant to 6TG.

2.2 Because HGPRT is an enzyme of the purine nucleotide salvage pathway, loss of the enzyme is not a lethal event. Different types of mutational events (base substitutions, frameshifts, deletions, some chromosomal type lesions, and so forth) should theoretically be detectable at the hgprt locus. The CHO/HGPRT assay has been used to study a wide range of mutagens, including radiations (2-4), and a wide variety of chemicals (1), and complex chemical mixtures (5).

1.1 This guide highlights some of the more relevant biological concepts as they are currently understood, and summarizes the critical technical aspects for acceptable bioassay performances as they currently are perceived and practiced. The Chinese hamster ovary cell/hypoxanthine guanine phosphoribosyl transferase (CHO/HGPRT) assay (1) 2 has been widely applied to the toxicological evaluation of industrial and environmental chemicals.

1.2 This guide concentrates on the practical aspects of cell culture, mutagenesis procedures, data analysis, quality control, and testing strategy. The suggested approach represents a consensus of the panel members for the performance of the assay. It is to be understood, however, that these are merely general guidelines and are not to be followed without the use of sound scientific judgement. Users of the assay should evaluate their approach based on the properties of the substances to be tested and the questions to be answered.

1.3 Deviation from the guidelines based on sound scientific judgement should by no means invalidate the results obtained.

1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.