ADME: An essential component of the safety assessment of nanoscale food ingredients
Nanotechnology has the potential to transform the entire food industry by changing the way food is produced, processed, packaged, transported and consumed. 1 Nanomaterials are generally defined as those classes of materials that have one or more physical dimensions at the nanoscale— ranging from 1 to 100 nanometers. 2 At such a small scale, nanomaterials exhibit different physical and chemical properties than larger sized versions of the same substance. 3 Compared to larger sized particles, nanoparticles have vastly increased surface-to-volume and surface-to-mass ratios. Further, as the size of a particle approaches the nanoscale, quantum mechanical effects of subatomic particles may produce fundamental changes in atomic structure, physical and/or optical characteristics, shape, solubility, thermodynamics, agglomeration properties, surface charge, antigenicity and/or reactivity. These changes may impart unique biological properties to materials, resulting in altered absorption, distribution, metabolism or excretion (ADME) in the body. As the toxicity of a substance is dependent on its ADME, the toxicological profile of a nano-sized material may be entirely different from the same material in a larger scale version.
While the gastrointestinal (GI) tract contains a variety of protective mechanisms that prevent entry of potentially toxic substances into the cells lining the GI tract (e.g. stomach acid, mucus, tight junctions between epithelial cells, and lymphoid tissue), it is designed to let beneficial substances in through diffusion or active transport. One of the primary concerns regarding oral exposure of nanomaterials is increased potential for permeability through cell membranes or tight junctions between cells, resulting in increased absorption into the blood. Once in the bloodstream, the nanomaterial may readily distribute to, or accumulate in organs. Nanomaterials may persist in the body, due to increased stability or decreased solubility in blood or tissues. Further, they could gain access to organs (e.g. brain or placenta) that ordinarily block entry of the larger form of the same material or organs of the reticuloendothelial system (e.g. liver, thymus or spleen) that contain resident phagocytic cells. Depending on dose and surface properties, phagocytes could effectively clear nanomaterials or be activated to produce toxicity. Thus, nanoparticles may exhibit unique toxicities that are due to not only the dose, but also to any alterations in the delivery of the substance.
Although in vitro testing may provide some information about the potential for nanomaterials to enter cells, in vivo ADME testing should be performed to assess the potential for absorption, distribution and/or accumulation in tissues. In contrast to soluble chemicals, nanoparticles can sediment, aggregate/agglomerate or bind to proteins, influencing their uptake into, and clearance from cells. Studies that have been performed to date indicate that there may be a dynamic equilibrium between those biological processes that induce accumulation of the nanomaterial and those that ultimately dispose of the material. 4 In vitro studies simply cannot examine this equilibrium.
ADME testing should be performed prior to in vivo toxicity testing because the data can be used to set doses or endpoints to be examined in in vivo toxicity tests. If results of ADME studies show that the nano-sized substance is not absorbed from the GI tract, in vivo genetic toxicity testing may be unwarranted. Alternatively, if the ADME of the nano-sized substance is found to be similar to that of the same substance in a conventional form, historical toxicity data from the conventional form could potentially be used to provide evidence of safety of the nano-sized material.
The Food and Drug Administration (FDA) is currently evaluating the use of nanomaterials on a case-by-case basis and has not developed any formal guidance for tests that should be performed to assess safety of nanomaterial food ingredients. Such guidance has been published by The European Food Safety Authority (EFSA). 5 The EFSA document emphasizes the importance of performing ADME studies prior to other in vivo studies and provides some practical solutions for performing ADME studies on nanoscale food ingredients. To maximize the utility of these studies, EFSA recommends the following:
Whenever possible, consider use of an aqueous dosing solution first. Possible interactions with the vehicle should be determined in advance, before in vivo The same vehicle should be used for further testing.
Administer by gavage in order to overcome dosing obstacles (e.g. tendency for agglomeration or absorption to walls of food or drinking water containers). Gavage administration can provide for delivery of a fairly precise, uniformly mixed dose.
Conduct a pilot study to identify doses and parameters to be examined in the main study. The dose should be sufficient to allow for identification of the nanomaterial in excreta and blood or plasma. Blood samples should be taken at regular intervals up to 24 hours after administration and nanomaterial retention in the gut epithelium and other organs and tissues should be investigated.
Use at least two doses in the main study, to correct for the possibility of effects associated with peak concentration rather than total exposure and to allow for proper design of repeated dose toxicity studies.
Consider performing comprehensive mass balance studies and/or repeated dose administration to obtain information on possible accumulation.
Use a method of detection that will detect the nanomaterial or its elemental composition. The choice of the detection technique should be based on the composition of the nanomaterial and the potential for release of various types of labels (e.g. radioactive or fluorescent). In addition, the impact of the labeling system on the properties and activity of the nanomaterial should be considered.
Additional guidance on the performance of ADME studies for complex nanomaterial constructs (e.g. substances encapsulated in polymeric nanoparticles or covalently bound to nanocarriers) can be found in an article published by Zolnik and Sadrieh. 6 Although the article is written from the drug perspective, it contains some helpful information about the conduct of ADME studies for orally administered nanomaterials. If the substance is expected to be released from the nanocapsule (or carrier) in the GI tract, ADME studies should be designed to distinguish the free substance from the nano-encapsulated (or carrier-bound) substance, and the nanocapsule (or carrier). Further, if there is any possibility that the nanocapsule or carrier and the substance may have different distribution profiles, studies with dual radiolabeling of the capsule or carrier and substance are warranted.
To conclude, an ADME study is an essential component of the safety assessment of nanoscale food ingredients. The benefits of performing an ADME study prior to other in vivo studies are well worth the cost, as the results can be used to design a testing strategy that will minimize use of unnecessary tests but accurately assess the potential for long term toxicity. Absorption and distribution to non-target organs as well as persistence of the nanomaterial are general indicators for in-depth in vivo testing. However, if results of the ADME study show that the material exhibits similar behavior as the same material in a larger scale version, historical toxicity data from the conventional form could potentially be used to provide evidence of safety of the ingredient at the nano-size.
References
Srinivas, P. R., Philbert, M., Vu, T. Q., Huang, Q., Kokini, J. L., Saos, E. Chen, H., Peterson, C.M., Friedl, K. E., McDade-Ngutter, C., Hubbard, V., Starke-Reed, P., Miller, N., Betz, J.M., Dwyer, J., Milner, J. and Ross, S.A. Nanotechnology research: Applications in Nutritional Sciences. J Nutr. 2009.
United States Food and Drug Administration. Considering Whether an FDA-Regulated Product Involves the Application of Nanotechnology (Draft). 2011. Available at: http://www.fda.gov/RegulatoryInformation/Guidances/ucm257698.htm.
Council of Canadian Academies. Small is different: A science perspective on the regulatory challenges of the nanoscale. 2008. Available at: http://scienceadvice.ca/uploads/eng/assessments%20and%20publications%20and%20news%20releases/nano/(2008_07_10)_report_on_nanotechnology.pdf.
Maynard, A. D., Warheit, D. B. and Philbert, M. A. The new toxicology of sophisticated materials: Nanotoxicology and Beyond. Sci. 2011;120(S1):S109-S129.
European Food Safety Authority (EFSA). Guidance on risk assessment on the application of nanoscience and nanotechnologies in the food and feed chain. EFSA Journal. 2011;9(5):2140.
Zolnik, B. S. and Sadrieh, N. Regulatory perspective on the importance of ADME assessment of nanoscale material containing drugs. Drug Delivery Reviews. 2009(61):422-427.