Oligomerisation products of beta-pinene

Endpoint summary

Description of key information

Standard information required at REACH Annex VIII to IX is available for assessing the toxicokinetics for the test substance alongside some supporting information on close phytoterpene analogues. 

 

The available test data do not permit extensive conclusions concerning absorption, metabolism or excretion to be conclusively drawn, although data indicate the substance is of low toxicity.

Additional information

Although no toxicokinetic studies have been performed on pinene oligomers, there is a large body of information on uptake, metabolism and excretion of close phytoterpene analogues.

 

Falk et al (1990: supporting study record) studied the toxicokinetics of alpha-pinene in human volunteers exposed by inhalation during two hours of light exercise, on four occasions: three (+)-alpha –pinene exposures (10, 225, and 450 mg/cu.m) plus one (-)- alpha –pinene exposure (450 mg/cu.m)). Relative pulmonary uptake averaged 59% at 225 and 450 mg/cu.m, with absolute uptake increasing linearly with increasing (+)-alpha –pinene concentration. Blood clearance post exposure was triphasic, with half-lives of 4.8-5.6, 38-40 and 695-555 minutes in the three phases: over 21h post-exposure the clearance rate was high at 1.1 l/h/kg. Elimination of pinene by respiration after the exposure period amounted to 7.5-7.7% of uptake, and urinary elimination of unchanged pinene was minimal (<0.001% of uptake during 30 minutes post-exposure, then undetectable. After exposure was terminated, less than 0.001% of the total uptake was eliminated unchanged in the urine and about 8% in exhaled air. A long half-life in poorly perfused tissues indicated a high affinity to adipose tissues. No acute changes in lung function were seen during or immediately after exposure.

No differences in uptake, distribution or elimination were seen between the two alpha-pinene isomers investigated. It was concluded that the observed high rate of clearance indicated pinene is readily metabolised (presumably via hepatic metabolism).

 

Kodama et al (1976: supporting study record) studied the metabolism of d-limonene in various species, concluding that its absorption from the gastrointestinal tract was both rapid and near-complete in animals. This conclusion was based on rapid bioelimination of radioactivity following oral administration of radiolabelled d-limonene: 75-95% of administered radioactivity was excreted in urine and <10% in faeces (animal tests) over 2-3 days post-dosing. Rapid bioelimination, with no significant accumulation of d-limonene or its metabolites, was concluded in all of these species.

Some differences in major metabolites were seen between species, with 40-65% of the administered material being accounted for. Possible routes of d-limonene metabolism involve oxidation and/or hydroxylation. In the rat, rabbit and hamster oxidation of the C1 methyl group forming perillic acid is predicted while in dogs and man diol formation at the C8-9 double bond is predicted: glucuronide formation and urinary excretion then follows.

 

The toxicokinetics and metabolism of d-limonene in both animals and man has also been reviewed by Falk Filipsson, Bard and Karlsson (1998: supporting study record). These authors noted work showing:

- rapid blood clearance in man following 2h exposure at 450 mg/cu.m (rate 1.1 l/kg/h, as noted earlier for alpha-pinene)

- high initial radioactivity in rat liver, kidneys and blood after oral dosing with radiolabelled d-limonene followed by reduction to negligible levels within 48h

- sex-related differences in rats, with higher d-limonene equivalent concentrations and some 40% reversibly bound to alpha-2-globulin (male rat specific, renal protein) in males

- 25-30% of orally administered d-limonene being found in human urine, in the forms of limonene diol and its glucuronide, after oral intake

- identification of perillic acid as the principal d-limonene metabolite in rat and human plasma

- limited available information concerning skin penetration, but one report of rapid dermal absorption of tritiated d/l-limonene from bathing water and one unquantified report of low d-limonene dermal uptake compared to that via inhalation.

 

IARC (1999: supporting study record) summarised additional in vitro and human studies of d-limonene metabolism:

- in vitro, rat liver microsomes converted d-limonene to limonene diols via an epoxidation step under conditions of alkaline, but not neutral, extraction

- following ingestion of 100 mg/kg d-limonene, blood samples drawn from 5 healthy human volunteers were shown (by GC-MS analysis) to contain three major metabolites (perillic acid, dihydroperillic acid and limonene diol) plus two minor ones (methyl esters of the acids). Rapid metabolism was indicated by the finding of higher metabolite concentrations 4h post-dose than after 24h

- blood samples taken from 32 patients with advanced solid tumour, metastatic cancers given on average three treatment cycles of oral d-limonene administration (17 females, 15 males: treatment cycle one dose, then after 2 days without treatment three doses daily for 18 days), maximal plasma concentrations of d-limonene were seen 1-6 h post-dose. The principal metabolites found were perillic acid, dihydroperillic acid and limonene diol plus uroterpinol and a perillic acid isomer. No accumulation of d-limonene or its metabolites was seen.

<< IARC, 1999. Monographs Volume 73 - d-Limonene >>

 

JiDong Sun (2007: supporting study record) also included a summary of human d-limonene absorption, metabolism and excretion in a review of its safety and clinical applications.  This reported work which showed:

- a half-life of d-limonene in humans of 12-24h

- peak plasma concentrations of perillic acid only 1h after ingestion of d-limonene (447-596 mg) in lemonade by healthy human volunteers, dropping to below the limit of detection within 24h.

 

A variety of phytoterpenes , including d-limonene, have been investigated as skin penetration enhancers for use in transdermal drug delivery systems: the review paper of Sapra, Jain and Tiwary (AAPS Journal 10(1): 120 -132, 2008) concluded that (at least for some drugs) they have been shown to aid dermal uptake.

 

Given the larger molecular size of pinene oligomers compared to limonene and pinene, slower and/or less extensive absorption, particularly via dermal penetration, can be predicted. The contrast between Guinea pig sensitisation test results when pinene oligomers was injected subcutaneously or applied topically for induction supports the hypothesis that its penetration through intact skin is limited.  However after absorption via any route rapid biotransformation by epoxidation to form diol metabolite(s), followed by glucuronidation and urinary excretion, can be expected.

 

In the available OECD 407 and 408 studies with the substance no adverse effects were observed at the limit does of 1000 mg/kg/day. However, there were indications in both studies of hepatic effects linked to adaptative changes in the liver/thyroid. This is indicative of absorption, distribution and metabolism potential for the substance.

 

In the OECD 414 study no adverse effects were observed at the limit dose of 1000 mg/kg/day although some changes to thyroid hormones could also potentially be linked to an adaptive liver response. This also indicates some potential of the substance for adsorption, distribution and metabolism.