Toxicokinetics

Absorption

Ochratoxin A is readily absorbed from the upper gastrointestinal tract, whereby the small intestine, particularly the proximal jejunum, has been found to be the major site of absorption [94]. Absorption from the duodenum can take place against a concentration gradient [94], highlighting the presence of organic anion transporter (OAT) proteins for OTA transport [95-97]. The acidic moieties (the carboxylic and the phenolic group) play an important role in OTA absorption. In most animal species OTA is passively absorbed in both its non-ionised and monoanion (OTA⁻) forms from the stomach and, particularly, from the proximal duodenum, which, in addition, is favoured by the high binding of OTA to plasma proteins [94, 98, 99]. The overall percentage of OTA found to be absorbed were 66 % in pigs, 56 % in rats and rabbits as well as 40 % in chickens [100, 101]. OTA is subsequently transported via the portal vein to the liver and thereafter distributed to different tissues and organs. The biological half-life of OTA in male Wistar rats was estimated to be 127 h [102]. The toxicokinetics of the mycotoxin when given intratracheally, intravenously and orally were comparable. Interestingly, simultaneous administration of OTA and phenylalanine to mice appeared to increase the absorption of OTA from the stomach and intestine and to increase gastrointestinal transit [99]. The bioavailability of OTA after oral administration was very low in fish, but ranged from 44 to 97 % in rat and mouse [103]. Moreover, bioavailability may vary in the presence of food components as well as inhibitors. OTA absorption and cellular accumulation is increased by some flavonoids and resveratrol [104].

Distribution

In the bloodstream more than 99% of OTA is bound to serum albumin and other macromolecules. The pronounced binding of OTA to plasma proteins retards its elimination by limiting its transfer from the bloodstream to hepatic and renal cells; however, OTA is only

detected in traces in erythrocytes [101, 103, 105-109]. In contrast, more specific binding of the toxin to small serum proteins (20 kDa), which can be filtered through the glomerular membrane, could be relevant for the nephrotoxic effects in mammals [110]. In vivo studies reveal that the lifetime of OTA is dependent on the presence of serum albumin and that this binding is species- and sex-dependent [108, 111]. Considerable variations in the serum half-life of OTA after oral administration have been reported: 4.1 h in chickens, 6.7 h in quail, 24–

39 h in mice, 77 h in pre-ruminant calves, 55–120 h in rats, 72–120 h in pigs, 456–504 h in vervet monkeys (Cercophithecus aethiops) and 510 h in rhesus macaques (Macaca mulata);

these are even higher after intravenous administration [98, 101, 103, 112-115]. In one human volunteer the half-life of OTA was 840 h [89]. The wide species difference in serum half-life of OTA was recently reviewed by Petzinger and Ziegler [89].

The concentration of ochratoxin A and its metabolites in tissues and plasma is dependent on the animal species, the dose and form of OTA administered (crystalline or naturally occurring in feed), the diet composition as well as the health status of the animals [93]. In general, the elimination half-life of OTA in blood is longer than in tissues, which may at least in part be attributed to the higher binding affinity of the mycotoxin to blood proteins [43, 103, 116]. In pigs, rats, chickens and goats OTA was mainly distributed to the kidneys with lower concentrations found in liver, muscle and fat [114, 117-119]. Moreover, it has been shown that tissue distribution is dependent on dose, sex and duration of OTA exposure (ingestion, injection) [120-123]. In mice, rats, pigs and humans it has been shown that OTA can readily cross the placenta, depending on the day of gestation; these suggest that the transfer is influenced by the developmental stage of the placenta, which is considered to be completely developed after day 9 of gestation [112, 124-129]. Moreover, a very efficient transfer of OTA from the blood to the milk and subsequently to the offspring has been demonstrated in lactating rats and rabbits [127, 130, 131]. Consistent with these observations is the fact that the amount of OTA found in kidney and blood of the offspring exposed both through the placenta and lactation were four- to five-fold higher when compared to offspring exposed only through the placenta or through lactation [127].

Elimination

Ochratoxin A shows a rapid absorption and a slow elimination. In all species studied both biliary and renal excretion have been shown to be involved in the plasma clearance of ochratoxin A. The relative contribution of each excretory route is influenced by the route of administration, the dose, the degree of serum macromolecular binding as well as the differences in the degree of enterohepatic recirculation of OTA [43, 69, 103, 132, 133]. The major excretory products are ochratoxin α (OTα), ochratoxin A itself as well as the 4R-Hydroxy-OTA epimer [134]. All the metabolites are considered to be less toxic than OTA itself. In addition to the urinary und faecal routes of excretion, the mammalian milk excretion appears to be relatively effective.

In humans and non-human primates (vervet monkeys) the parent OTA is mainly excreted via the kidneys [115, 116]. Due to the high plasma protein binding potential of OTA its glomerular filtration is limited. Instead, OTA rather undergoes tubular secretion into the urine and is also reabsorbed in all nephron segments [135-137]. The involvement of several transporter proteins carrying OTA across tubular membranes as well as their sex-dependent regulation has been recently reviewed by Pfohl-Leszkowicz and Manderville [93]. The reabsorption of filtered and secreted OTA retards its excretion and may lead to the accumulation of the toxin in the renal tissue and thus contribute to its renal toxicity.

Faecal excretion of OTA is mainly due to biliary excretion and very efficient [94, 99, 138].

Furthermore, the amount of OTA secreted by the intestinal epithelial cells into the lumen of the intestine must also be taken into account and is comparable to the amount of OTA transported via the bile into the intestine [94, 100, 139]. In the intestine the conjugated compounds are hydrolysed by the intestinal microflora, and the released OTA may undergo enterohepatic circulation. The microbial microflora has also been shown to hydrolyse OTA to OTα in the large intestine [92].

Another important route of excretion of OTA is the elimination via the milk of lactating mammals. Several studies have been carried out in different mammalian species to evaluate the transfer of orally or intravenously administered OTA and of OTA present in contaminated feed into milk [127, 130, 131, 140-143]. Depending on the dose originally administered or present in the feed OTA and OTα have been identified in milk. Several authors have reported on the OTA levels in human milk, thereby concluding that OTA amounts in human milk show

significant inter-individual and geographic variations [143-145]. More recently, a correlation between OTA contamination of human milk and its dietary intake has been shown by Skaug et al. [146].