512 IARC MONOGRAPHS VOLUME 92 4. Other Data Relevant to an Evaluation of Carcinogenicity and its Mechanisms 4.1 Toxicokinetics 4.1.1 Absorption, distribution, metabolism and excretion (a) Overview This section provides an overview of the toxicokinetics of polycyclic aromatic hydrocarbons (PAHs). Other more comprehensive reviews of the toxicokinetics of PAHs include those by the Environmental Protection Agency (1991), the Agency for Toxic Substances and Disease Registry (ATSDR, 1995) and the International Programme on Chemical Safety (IPCS, 1998), as well as reviews on the metabolism and bioactivation of PAHs by Conney (1982), Cooper et al . (1983), Shaw and Connell (1994), Penning et al . (1999), Joint FAO/WHO Expert Committee on Food Additives (JECFA, 2005) and Xue and Warshawsky (2005). Little is known about the toxicokinetics of mixtures of or individual PAHs in humans. Multiple studies have been conducted to monitor urinary metabolites of PAHs and PAH–DNA adducts in the lymphocytes of workers exposed to mixtures of PAHs. However, most of the available data on toxicokinetic parameters for PAHs derive from studies of benzo[ a]pyrene in animals. Because of their lipophilicity, PAHs dissolve into and are transported by diffusion across lipid/lipoprotein membranes of mammalian cells, thus facilitating their absorption by the respiratory tract, gastrointestinal tract and skin. PAHs with two or three rings can be absorbed more rapidly and extensively than those with five or six rings. Once absorbed, PAHs are widely distributed throughout the body, with some preferential distribution to or retention in fatty tissues. They are rapidly metabolized to more soluble metabolites (epoxides, phenols, dihydrodiols, phenol dihydrodiols, dihydrodiol epoxides, quinones and tetrols), and conjugates of these metabolites are formed with sulfate, glutathione (GSH) or glucuronic acid. The covalent binding of reactive PAH metabolites to form DNA adducts may represent a key molecular event in the formation of mutations and the initiation of cancer. The structures of the DNA adducts that are formed provide an inference of the precursor metabolites. PAHs are eliminated from the body principally as conjugated metabolites in the faeces, via biliary excretion, and in the urine. Most PAHs with potential biological activity range in size from two to six fused aromatic rings. Because of this vast range in molecular weight, several of the physicochemical properties that are critical to their biological activity vary greatly. Five properties in particular have a decisive influence on the biological activity of PAHs: their vapour pressure, their adsorption onto surfaces of solid carrier particles, their absorption into liquid carriers, their lipid/aqueous partition coefficient in tissues and their limits of solubility in the lipid and aqueous phases of tissues. POLYCYCLIC AROMATIC HYDROCARBONS 513 These properties are intrinsically linked with the metabolic activation of the most toxic PAHs, and an understanding of the nature of this interaction may facilitate the interpretation of studies on their deposition and disposition that are occasionally conflicting. Transporters may play a role in the biological activity of PAHs. Adenosine triphosphate (ATP)-binding cassette (ABC) transporters (49 genes characterized in humans) transport specific molecules across lipid membranes including hydrophobic compounds and metabolites (Schinkel & Jonker, 2003). P-Glycoprotein transports mainly non-metabolized compounds and multidrug resistance-associated protein-1 (MRP1) and -2 conjugates of foreign compounds (Leslie et al. , 2001; Haimeur et al. , 2004). Several ABC transporters are polymorphic (Sakaeda et al. , 2004). Benzo[ a]pyrene conjugates may be substrates for ABC transporters, such as the GSH conjugate of benzo[ a]pyrene- 7,8-diol-9,10-oxide which is a substrate for MRP2 (Srivastava et al. , 2002) and benzo- [a]pyrene-3-glucuronide which is a substrate for breast cancer resistance protein (BCRP) (Ebert et al. , 2005). PAHs are ubiquitous in the environment due to volcanic eruptions and forest fires, and their presence in various media (i.e. air and soil) constitutes a background level of exposure. Additional exposure occurs through the ingestion of grilled or cured meats (see Section 1). These exposures should be taken into consideration when assessing health risks due to exposure to PAHs. (b) Absorption through the respiratory tract Vapour pressure is a major determinant for the distribution of a PAH between the particulate and gaseous phase of the aerosol by which the substance is emitted into the atmosphere. The vapour pressure of PAHs decreases drastically with increasing molecular weight (Lohmann & Lammel, 2004), so that two-ringed naphthalenes are mostly found in the gas phase whereas five-ringed PAHs such as benzo[ a]pyrene are mostly adsorbed on airborne particles at room temperature (Lane & Gundel, 1996). Strong sorption of a PAH onto particles can further increase the particle-bound fraction of that substance (Lohmann & Lammel, 2004). Because the most carcinogenic PAHs of greater size are, to a large degree, particle-associated, there is considerable potential for covariance with an inflammatory response that is induced by the carrier particles alone. Typical carbonaceous carrier particles of PAHs that have no adsorbed genotoxic material have been shown to be carcinogenic, particularly in rats (see IARC, 2010). This mechanism is, however, outside the scope of this monograph. Gas/particle partitioning is also of great importance during inhalation exposure in order to determine the probable sites of deposition within the respiratory tract. The smaller gaseous PAHs are deposited mostly as soluble vapours, whereas five- to six- ringed aromatic compounds are mostly particle-associated at ambient temperatures and can be expected to be deposited with the carrier particles. The rate and extent of absorption by the respiratory tract of PAHs from PAH-containing particles are dependent on particle size (i.e. aerodynamic diameter, which influences regional deposition in the respiratory tract) and the rate of release of PAHs from the particle. Because the release of PAHs from particles is extraneous in 514 IARC MONOGRAPHS VOLUME 92 exposure to vapours, the rate and extent of absorption of inhaled vapour-phase PAHs are different from those of particle-bound PAHs. After deposition in the respiratory tract, the sorptive properties of PAHs are a major determinant for the bioavailability of the substance in the organism. The timing of the release from carrier particles in particular affects the toxicity of inhaled PAHs at the site of entry. For solid particles, the major determinant for the release is the rate of desorption of the hydrocarbons from the surface, whereas for liquid aerosols, either the dissolution of the entire particle or desorption from insoluble carrier particles is a decisive factor. A rapid release from carrier particles gives a close correlation between the deposition pattern of inhaled aerosols and the site-of-entry exposures to particle-associated PAHs. Slower release alters the exposures, and shows a clearance pattern of inhaled particles. Substantial fractions of inhaled PAHs deposited in the tracheobronchial region and upper airways can be redistributed by the mucociliary escalator to the gastrointestinal tract, which thereby changes the exposure route from inhalation to ingestion (Sun et al ., 1982). Following deposition and desorption from their carrier particles, PAHs are absorbed through the epithelial barriers onto which they are deposited. The slow diffusion of highly lipophilic substances into the tissues is fundamental to the behaviour of PAHs in biological systems. This is a strictly physicochemical mechanism that needs to be considered in all measurements of the kinetics of PAHs in tissues. A highly lipophilic substance dissolves readily in the first lipid membrane it encounters, but is then transported slowly into the next layer (Gerde et al ., 1993a). This is due to the low concentration of lipophilic solute in the aqueous gaps between cell membranes, which results in a high concentration of solute in the epithelium at the site of entry, comparatively slow absorption into the circulation and a low concentration in all tissues distal to the site of entry. The absorption process is strongly dependent on the lipophilicity of the PAHs and their metabolites: with higher lipophilicities, the mobility of the substances by diffusion into tissues is lower and, with thicker entrance epithelium, the half-life of absorption into the capillary bed of the submucosa is longer. Highly lipophilic PAHs that are released from particles deposited in the conducting and bronchial airways are retained for several hours and absorbed slowly by a diffusion-limited process, whereas PAHs that are released from particles in alveolar airways are absorbed within minutes (Gerde et al ., 1991a,b; 1993a,b,c; Gerde & Scott, 2001). The relative thickness of the epithelium of the conducting airways compared with the thin epithelium of the alveolar region has been proposed as a contributing factor to this regional difference in duration of absorption following deposition. Slow absorption through the epithelium of the conduct- ing airways probably leads rapidly to saturation in the mucous lining layer and airway epithelium with increasing levels of exposure (Gerde et al. , 1991a,b). A probable consequence is an increase in the fraction of undissolved/undesorbed PAHs