Strengths:
Cell-Process assays capture endpoints that may be missed by the previous two tiers, for example nongenomic actions of nuclear hormone receptors. Therefore, they are valuable and could be performed in preference to computer based and target cell assays if budget is limited and broad answers are required. Assays in this tier are cell-process based; therefore, they can interrogate integrated pathways by measuring function and can be more sensitive for detecting endpoints than receptor specific assays. For example, BPA is a poor estrogen in receptor-based assays but a very strong estrogen in Cell-Process proliferation assays and in non-genomic signaling assays.
Cell-Process assays can provide a higher level analysis of compound action since many potential routes of, for example, estrogenicity can be interrogated in a single assay (nuclear estrogen receptor activity, membrane estrogen receptor activity, alterations in the levels of endogenous estrogens or of co-factors required for receptor activity). Therefore, the sensitivity of Cell-Process assays may be higher in some cases than target based assays in other tiers.
Limitations:
Cell-Process assays might not reveal complete details of the mechanism of action. For example, knowing that a chemical was estrogenic because it promoted proliferation of estrogen-dependent cancer cells would not necessarily reveal just how the chemical acted (i.e., which receptor it acted on and how).
Cell-Process assays are not currently available for all possible endocrine endpoints and are not a substitute for assays that measure the activity of chemicals, in vivo, on an integrated living system rather than isolated cells. Since not all possible cell types are represented, whole animal assays in the Fish/Amphibian and Mammalian tiers might well be required to capture all possible actions.
Additionally, only some cell-process assays are commercially available and we have not vetted the existing suppliers of assays. Technical care is required to successfully execute these assays, necessitating the development of robust assays that employ multiple positive and negative control chemicals (see TiPED Guiding Principles). While it is unlikely that chemicals positive in this Tier will be negative in whole animal tiers, it is possible that chemicals which are negative in this tier could have effects in whole animal tiers.