2. Pharmacokinetics of catechins

Minor constituents include caffeine, theobromine, and theophylline. Among green tea catechins, EGCG is the most abundant and biologically active based on animal and human studies [1, 2].

Health benefits are evident for green tea catechins including anticancer [4] and antimicrobial activities [5]. Cancer prevention has been found in the colon, duodenum, esophagus, stomach, large intestine, liver, lung, mammary glands, and skin [6, 7]. There were in vitro anticancer effects on adult T-cell leukemia (ATL) caused by a latent infection of human retrovirus HTLV-1 [8]. Drinking green tea could reduce HTLV-1 provirus load in asymptomatic HTLV-1 carriers [9]. Although the underlying mechanisms for cancer-prevention is not well understood, their antioxidation, free radical scavenger activities [10], and NF-κB inhibitory effect [11] have been

Although biological and intervention studies have indicated various beneficial effects, results of clinical studies are not conclusive [12, 13]. Inconsistent findings may be attributed to variations of methodologies and study conditions such as differences in infusion techniques, consumption behavior, production methods, compositions of green tea constituents, and absorption profiles, among different study cohorts. Pharmacokinetics of green tea catechins have been studied using defined green tea catechin extract (GTE), Polyphenon E, and EGCG with variable doses of administration [14, 15]. Oral bioavailability of tea catechins in human plasma was found having 5 to 50 times lower than the level needed to exert biological activities in in vitro systems [16, 17]. Other studies showed very high free EGCG peak plasma concentrations, 300, 1970, and 2020 ng/mL, after ingestion of 3, 5, or 7 capsules of Sunphenon DCF-1 (containing 225, 375, and 525 mg EGCG), which is a green tea extract obtained by a defined protocol [18, 19]. Dosage affects the bioavailability of catechins. Many studies showed large

The biological activity is attributed to its structure moiety and hydroxyl groups [3].

18 Pharmacokinetics and Adverse Effects of Drugs - Mechanisms and Risks Factors

attributed as the major factors.

Figure 1. Structures of catechins.

It is essential to know the effective concentrations and forms of catechins present in plasma and tissues after ingestion. Pharmacokinetics is a study of the absorption, distribution, metabolism, elimination, and bioavailability of catechins following administration. In brief, after oral administration of green tea or extract, catechins are absorbed from the small intestine and remaining excess catechins pass to the colon. In the small intestine, catechins are conjugated with glucuronic acid, sulfate or by O-methylation. Some catechins with secreted bile in an enterohepatic recirculation process pass into the colon and are degraded into different flavonoid rings by resident microorganisms. The catabolized phenolic acids can be reabsorbed into the circulation and excreted into urine. Catechins, their conjugated metabolites, and a large amount of catabolized small phenolic acids can be distributed to various organs and tissues, absorbed by tissue cells, further metabolized, and perform various biological actions (Figure 2).

Figure 2. Schematic diagram showing the absorption, distribution, metabolism, elimination of catechins in the body. Thick and fill arrows indicate the flow of large amount of colon metabolites. Thin and hollow arrows depict low amount of present catechins flowing in the body.
