Nature of the Radiolabelled Test Substance for Registration Studies

Radiolabelled compounds are routinely used to generate data required for registration of pesticides, small molecule pharmaceuticals and other chemicals, under Guidelines published by the OECD and the OCSPP (EPA).

In many cases the use of radiolabelled material (PAIRA-Pure Active Ingredient, Radiolabelled) is considered mandatory and the choice of radiolabel is normally carbon-14. In some cases tritium may be used but its use is discouraged, or even prohibited, as hydrogen-tritium exchange may occur rendering interpretation of data difficult.

Reasons for choosing carbon-14

  • 14C is the regulatory isotope of choice
  • It has a long half-life –thus no requirement to correct for decay in extended studies
  • Defined label position(s) in the core structure of the molecule
  • Compounds can be prepared using tailored custom synthesis
  • Provides a highly sensitive method for Detection which is Quantitative & Qualitative
  • Allows fate of parent and metabolites to be followed and pathways elucidated
  • Critical quality controls including GLP support its use in regulatory studies

Examples of Guidance which highlights the nature of the radiolabel and carbon-14 specifically:

Radio-labels shall be positioned at sites (one or more as necessary), to facilitate elucidation of metabolic and transformation pathways and to facilitate investigation of the distribution of the active substance and of its metabolites, reaction and breakdown products (EU 283/2013).

Data on metabolism together with a schematic diagram of the metabolic pathways in plants and animals is required and these studies shall be conducted with one or more radio- labelled forms of the active substance and, where relevant, stereoisomer forms of the active substance and its metabolites (EU 283/2013).

Radiolabels are required to allow quantification of total extractable and un-extracted residues and 14C is the preferred isotope for residue studies (OECD 501).

14C labelled material is a prerequisite for some studies (OECD 304A).

The 14C atom should be in the most recalcitrant portion of the molecule (OECD 314)

If compound has ring structures, then conduct experiments separately on each ring with no more than one ring structure labelled in each experiment (OCSPP 850 170 & 1730).

Separate studies reflecting labelling of each ring will normally be required (OECD 502) if cleavage of molecule occurs- in addition to radiolabel, use of stable isotopes is encouraged.

Radiolabel may be preferred for studies in Manure, where phenyl ring U-14C is especially preferred, as 14C CO2 and 14C volatiles such as 14CH4 must be trapped (EMA/CVMP 2009/430327).

14C should be used for all mass balance and metabolite identification unless sensitivity of analytical  specificity is at least as good as radiolabel sensitivity and provides adequate mass balance and met ID information. The label should be in the core portion of the molecule and multiple labelling may be necessary to follow the metabolic fate (OECD 417).

Use of a Radiolabel may be optional but is preferred for example in bioaccumulation studies as it can facilitate analysis (OECD 305, OECD 315 & 317).

Tritium is often strongly discouraged (OPPTS 860.1300), not to be used (SANCO/11187/2013) or prohibited (OECD 507).

14C is recommended in some tests (OECD 309).