Genotoxicity Testing

• Ames Test
• GreenScreen HC Genotoxicity Test

Improve Your Drug Screening Efficiency

Genotoxicity and carcinogenicity are common properties of chemicals. The Carcinogenic Potency Database, which has data from long-term rodent cancer tests on 1485 chemicals, has positive toxicity results for over 60% of all chemicals tested. It is not surprising that genotoxicity is the cause of failure for many in vivo studies and clinical trials.

Contact Apredica for assistance in identifying genotoxicity in your drug candidates.

Ames Testing

The Ames Test is required for FDA IND submission. While the Ames Test provides near-certain results that a compound is genotoxic, a negative result on the Ames Test does not assure that the compound is not genotoxic because the Ames Test uses bacteria which lack many of the genotoxin targets found in mammalian cells. The Gentronix GreenScreen Test for genotoxicity is substantially more accurate than the Ames Test. Unfortunately, the FDA has yet to recognize this. Consequently, Apredica recommends that compounds be screened first using GreenScreen, with Ames testing performed immediately prior to IND submission.

Apredica's Ames assay for determination of mutagenicity of test agents determines genotoxicity using the Ames MPF™ 98/100 Mutagenicity Assay.

Principle of the Ames Assay

Salmonella typhimurium strains TA100 and TA98 have point mutations that make them incapable of growing in medium unless histidine is supplied. Treatment with a genotoxic chemical causes a mutagenic event to occur, during which base substitutions or frameshifts within the His gene may cause a reversion to histidine prototrophy, and the mutated organisms become able to grow in histidine-deficient medium.

Ames Assay Protocol

Approximately 10⁷ TA100 or TA98 organisms are exposed to 6 serial dilutions of test agent at 37ºC for 90 minutes in medium containing sufficient histidine to support approximately two cell divisions. The cultures are diluted in pH indicator medium lacking histidine and aliquoted into 48 wells of a 384-well plate. The plates are incubated for 48 hours at 37ºC, then wells containing cells that have undergone the reversion to histidine prototrophy and have grown into colonies are counted for each dose and compared to a zero-dose (solvent) control. Each dose is done in triplicate to allow for statistical analysis of the data.

A two-fold increase in the number of revertant colonies upon exposure to test chemical relative to the zero-dose controls indicates that the chemical is mutagenic in the Ames MPF™ 98/100 assay.

A pre-screen determination of dose range can be performed to measure solubility and cytotoxicity of test agents.

The genotoxicity of metabolites of the test agents can be determined by performing the assay in the presence of liver S9 fractions.

Mutagenicity of Doxorubicin in Ames Assay

Need Ames testing? Contact Apredica.

GreenScreen HC Genotoxicity Test

Gentronix has developed a superior genotoxicity screening solution with its GreenScreen HC (Human Cells). GreenScreen HC's accuracy derives from its unique ability to both correctly identify genotoxins and to correctly identify non-genotoxins. Apredica is a Gentronix-Authorized Service Provider.

Validation studies¹ have shown that GreenScreen HC identifies all mechanistic classes of direct-acting genotoxins including aneugens, topoisomerase inhibitors and DNA synthesis inhibitors. It can significantly reduce the number of problem compounds in preclinical development, thus minimizing the need for expensive and time-consuming mechanistic studies.

It is also important to determine if a candidate compound is metabolized to a genotoxic agent. The GreenScreen HC assay can also be performed with a source of exogenous metabolizing enzymes (S9)² commonly used in genotoxicity testing to provide information on the pro-genotoxic properties of compounds.

Biology

GADD45a mediates the adaptive response to genotoxic stress. The patented GFP fluorescence reporter includes complex regulatory elements. The assay gives positive results for direct-acting mutagens, clastogens, as well as aneugens, and topoisomerase and polymerase inhibitors. Importantly, the assay gives correct negative results for non-carcinogens, many of which give misleading positive results in other in vitro tests.

GreenScreen Genotoxicity Assay Protocol

Nine, 2-fold dilutions of each compound together with positive controls are set out in the microplate and growing cells are added to each well. After incubation and measurement in a microplate reader, software provides automated decisions and a clear graphical output.

Contact Apredica for assistance in identifying genotoxicity in your drug candidates.

Footnotes

1. Validation of the GreenScreen HC GADD45a-GFP genotoxicity assay. Hastwell P.W. et al (2006) Mutation Research 607: 160-175.

2. Assessment of the genotoxicity of S9-generated metabolites using the GreenScreen HC GADD45a-GFP assay. Jagger C. et al (2008) Mutagenesis pp. 1-16.