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Administrative data

basic toxicokinetics
Type of information:
other: expert statement
Adequacy of study:
key study
1 (reliable without restriction)

Data source

Reference Type:
other company data
Report date:

Materials and methods

Objective of study:
Test guideline
no guideline followed
GLP compliance:

Test material

Constituent 1
Test material form:
solid: crystalline
Details on test material:
- Name of test material: Thymidine

Results and discussion

Toxicokinetic / pharmacokinetic studies

Details on absorption:
Thymidine is a white to off-white crystalline powder at ambient temperature. The substance has a molecular weight of 242.23 g/mol, exerts a low vapor pressure (8.84E-010 Pa at 25 °C), has a low logPow (< 0.3 at 25 °C) and is very soluble in water (43’870 mg/L at 20 °C). A pKa of 9.28 was determined. Particle size distribution analysis revealed a range of D10, D90 [63 μm, 500 μm] with 37.6 % < 100 μm and 1.5 % < 45 μm.
Thymidine may be inhaled as dust. Based on the particle size, Thymidine is more likely to settle in the nasopharyngeal region than in the tracheobronchial or pulmonary regions. Due to the low logPow direct absorption across the respiratory tract epithelium is not very likely. Instead, the high water solubility points towards retention within mucus. Thus, absorption via respiratory tract is expected to be low.
Upon exposure to the skin the substance may dissolve into the surface moisture of the skin. However, Thymidine may be too hydrophilic to cross the lipid rich environment of the stratum corneum. Thus, absorption via the skin is expected to be low.
Highest rates of absorption are expected via the oral route. Upon ingestion Thymidine is likely to dissolve in the gastrointestinal (GI) fluids. As a weak acid, with a pKa of 9.28, the main site of absorption most likely is the small intestine. Maximum plasma concentrations are expected within the first few hours following exposure.
Several studies on absorption using the structural analog L-Thymidine were carried out (EMEA, 2007). Orally administered doses of 10 mg/kg bw resulted in bioavailability of 60% in rats, 59% in monkeys and 38% in woodchucks, maximum plasma concentrations reached within 1 to 3 h. Studies conducted with fed and fasted female rats suggested that food had no effect on the absorption of L-Thymidine (10 mg/kg dose). L-Thymidine exposure was less than dose-proportional, especially at doses greater than 1000 mg/kg bw. As shown in an in vitro Caco-2 study, L-Thymidine has moderate permeability properties. Various in vitro studies demonstrated that L-Thymidine does not interact with P-gp and MRP. A lack of interaction between L-Thymidine and transporters was also shown in humans. Finally, there were no sex-related differences in the pharmacokinetics profile of L-Thymidine.
Details on distribution in tissues:
Following uptake and becoming bioavailable Thymidine is likely to distribute throughout the whole body water, tissues and brain. Takeda, et al. (1984) assessed the distribution of tritiated Thymidine in various tissues of rat 24 hours after oral administration of tritiated thymidine. Highest levels were found in the small intestine and the spleen. Lower levels were observed in the liver, lung, testis, kidney, brain and muscle. Following i.v. injection in mice, tritiated thymidine was apparently rapidly absorbed by all cells, leaving the blood stream within a few minutes and found in all tissues, with highest levels in kidney and liver (Hughes et al., 1958). In human pharmacokinetic studies with tritiated Thymidine plasma levels following i.v. injection reached uniform mixing throughout the total body water within several minutes (Rubini et al, 1960; Straus et al., 1977).
Oral administration of [14C]-L-Thymidine (10 mg/kg bw) to male rats also resulted in whole body distribution, with the highest concentrations observed in small and large intestines, urinary bladder, kidneys, prostate gland, mesenteric lymph nodes, stomach, and pancreas (EMEA, 2007). There was limited penetration to the central nervous system, moderate crossing the placenta, and substantial secretion into rat milk as evidenced by a milk/plasma AUC ratio of 2.3. L-Thymidine was weakly bound to plasma protein (from 3.3 to 7.5 % in rat, monkey and human) and was independent of L-Thymidine concentration over the range evaluated (0.4 to 40 μg/ml). L-Thymidine partitioned into the erythrocytes of rats, monkeys and humans independently of its concentration (range 32 to 43 %).
Details on excretion:
Based on its molecular weight, water solubility and pKa Thymidine and its metabolites are likely to be excreted via the urine. Zaharko et al. (1979) identified kidney clearance of intact Thymidine the primary route of removal at high plasma concentrations and noted that metabolism, which plays a major role in the clearance of Thymidine at micromolar concentrations in plasma, appears to be saturated at millimolar concentrations of Thymidine. CO2 produced during metabolism may be exhaled.
After a single oral administration of [14C]-L-Thymidine to rats (10 mg/kg) radiocarbon was eliminated in both urine and faeces in approximately equivalent amounts (40-50 %) over 168 hr; overall recovery of radiocarbon was > 91 % (EMEA, 2007). The comparison of results after both routes of administration suggested that urine was the primary route of excretion, and that radioactivity recovered in the faeces after oral administration corresponded to unabsorbed substance.
In monkeys, drug-related radioactivity was primarily eliminated in the urine following intravenous administration of [3H]-L-Thymidine (74 % of the dose as L-Thymidine). After oral administration, 36.6 % of the administered compound was eliminated in the urine.
L-Thymidine was eliminated at a moderate to rapid rate (t1/2 2-8 hr) in mice, rats, and woodchucks, but more slowly in monkeys (t1/2 7.5-18 hr) and humans (t1/2 41.1 hr). No gender differences were observed in elimination.

Metabolite characterisation studies

Details on metabolites:
Park and Mitra (1992) assessed the metabolism of Thymidine in the GI tract. Before becoming bioavailable Thymidine was rapidly metabolized into nucleobase and sugar in the upper tract. No metabolites appeared in the colon. Notably, a free 3'-OH group seemed required for the metabolism (catabolism) of thymidine analogues in the rat intestine mainly by pyrimidine nucleoside phosphorylase.
Bioavailable Thymidine may be rapidly incorporated into the DNA of proliferating cells. Following i.v. injection of tritiated Thymidine in humans up to 90 % was lost from the blood within the first minutes (Rubini et al., 1960). About one-half of the plasma tritiated thymidine is catabolized to tritiated water, while the rest was presumably incorporated into DNA or converted to degradation products.
The salvage pathway to Thymidine synthesis involves the enzyme thymidine kinase and in a first step leads to the formation of β-aminoisobutyrate. Ultimately degradation leads to the formation water and CO2 via the citric acid cycle. Fink et al. (1955) incubated Thymidine with rat liver and identified a number of metabolites via the formation of β-aminoisobutyric acid, including p-aminoisobutyric acid, 5-hydroxymethyluracil, alanine and glucose, as well as 5-hydroxymethyluracil, urea, dihydrothymine, P-ureidoisobutyric acid, uracil-5-carboxylic acid and riboside (5-methyluridine). All of the metabolites are practically non-toxic.
Notably, while the mono-, di- and triphosphate metabolites of L-Thymidine have been observed in vitro, they have not been seen in plasma, urine or faeces in vivo (EMEA, 2007). Following a single oral dose of radiolabelled L-Thymidine to rats, the parent compound was the predominant radioactive component excreted in both males and females. One minor and unidentified metabolite was observed in females. This metabolite represented up to 3.9% of the radioactivity present in plasma and no more than 7.3% of radioactivity in urine (representing no more than 1.1% of the administered radioactivity). As less than 0.80% of the total administered dose was eliminated in the bile, these were considered to be minor metabolites. No metabolites were observed in plasma or urine of monkeys or woodchucks receiving radiolabelled L-Thymidine.
L-Thymidine did not inhibit in vitro human CYP450 enzymes including CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4. In addition, L-Thymidine did not induce rat CYP1A2, CYP2B, 3A or 4A in vivo. The occurrence interactions with other compounds mediated via cytochrome P450 therefore seems unlikely.

Applicant's summary and conclusion

Interpretation of results (migrated information): no bioaccumulation potential based on study results
Thymidine is ubiquitously present in nature and exists in all living organisms. The substance is one of the pyrimidine deoxynucleosides and part of double-stranded DNA. Degradation of Thymidine ultimately leads to the formation of CO2 and water. The parent compound and its metabolites are practically non-toxic.
The main route of Thymidine exposure is via the oral pathway. Following oral uptake the substance is likely to solubilise in GI fluids and is absorbed via the small intestine, if not metabolised in the GI tract. Bioavailable Thymidine distributes throughout the body water, cells and tissues. Thymidine may then be incorporated into DNA by replicating cells or degraded via salvage pathways. Metabolism is mainly via thymidine kinase leading to the formation of β-aminoisobutyraten and ultimately resulting in the formation of water and CO2. Degradation by Cytochrome P450 metabolism seems unlikely. The substance is not activated and does not bioaccumulate. Elimination of the parent compound and metabolites is most likely via the kidneys and urine.

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