Evaluating the toxicity of escalating dose of oral picolinic acid in Sprague-Dawley rats
Introduction
The great majority (∼95 %) of dietary tryptophan is catabolized through the kynurenine (KYN) pathway. The process starts with the activation of the enzyme indoleamine 2,3-dioxygenase-1 (IDO-1), and the resultant KYN is subsequently converted into either quinolinic acid (QA) or picolinic acid (PIC). These two compounds have been linked with regulation of oxidative stress and cell differentiation (Guillemin, 2012; Jones et al., 2013; Vécsei et al., 2013; Braidy and Grant, 2017). There is compelling evidence that both endogenous and exogenous PIC have anabolic effects on bone in vivo and in vitro (Vidal et al., 2015; Duque et al., 2020). Also, human studies indicate that serum kynurenines and the KYN pathway metabolites are positively associated with the bone mineral density (BMD) of older adults (Apalset et al., 2014), and altered KYN pathway is associated with the progression of osteoporosis, which can be prevented by administration of etidronate (a bisphosphonate and bone antiresorptive) or raloxifene (an estrogen receptor agonist with positive effects on postmenopausal osteoporosis) (Forrest et al., 2006).
Recently, we showed that PIC has osteoanabolic properties in both normal and ovariectomized C57BL/6 mice, which at a dose of 0.25−1 g/kg/d for 6 weeks showed increased bone formation, bone mass, and bone mechanical strength (Duque et al., 2020). This occurred without any primary effects on mineral absorption or calciotropic hormones (Duque et al., 2020). These results suggest a therapeutic potential for KYN pathway products, particularly PIC, for treating and/or preventing osteoporosis. Hence, determining the toxic dose thresholds is essential for this therapeutic candidate to be tested on other animal models of osteoporosis or human clinical trials.
The development of new medications for treatment and prevention of osteoporosis and fragility fractures is a necessity as the majority (>60 %) of older adults above 65 show low BMD increasing to above 80 % in 80+-year-old people (Zanker and Duque, 2019). The two major classes of current osteoporosis medications (i.e. antiresorptives and anabolics) not only are costly, but are also associated with side effects. In addition to less serious side-effects common to oral osteoporosis medications (e.g. dysphagia, dyspepsia and gastric ulcer), antiresorptive drugs (mainly bisphosphonates and denosumab) are associated with osteonecrosis of the jaw (Bagan et al., 2016), and atypical fractures of the femur (Aspenberg, 2014). In addition, teriparatide - the most widely available bone anabolic - has been linked to the development of osteosarcoma in rats, which has limited the duration of its use as osteoporosis treatments (Gilsenan et al., 2018). Although uncommon to rare, such side-effects are of severe consequences and a reason for therapy reluctance, especially in older persons.
PIC, however, is affordable and may have the potential to be used as a primary or complementary medication for the treatment and prevention of osteoporosis and bone fragility. This pilot study investigated the clinical signs and main affected organs in PIC toxicity in a rat model. This would be essential for dose adjustment of PIC in the development of new drugs.
Section snippets
Animals
Sprague-Dawley male rats (Charles River, St. Constant, Quebec) were individually housed until randomization, after which the animals were assigned to study groups. Then the rats underwent a seven-day acclimatization period, during which they were tattooed on the tail for identification. Animal house rooms were kept at 19−25 °C temperature, 30–70 % humidity and 12 hourly dark and light cycles. The animals had ad libitum access to food (PMI Nutrition International Certified Rodent Chow No. 5CR4
Body weight
Changes in body weight before and after treatment are summarized in Table 1, Table 2, Table 3.
Serum biochemistry
Administration of PIC led to a PIC-dose/time-dependent increase in the serum concentration of all tested metabolites of the KYN pathway in the serum and CSF, indicating efficient administration and absorption of the chemical (Table 4). No significant abnormalities or associations with the dose of administered PIC were observed in the serum biochemistry of the investigated analytes (Table 5).
Discussion
Oral PIC 1000 mg/kg/day was proven to be acutely to per acutely lethal in two days. Consecutive consumption of 500 mg/kg for just a few days was also associated with severe toxicity, neurological signs, and sudden death. These higher doses were associated with central nervous system necrosis/gliosis, and/or hemorrhage as the main responding organ. Rats receiving lower doses (125 or 250 mg/kg/day) of PIC completed treatment course without apparent clinical adverse events.
Our findings are
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Gustavo Duque reports financial support, administrative support, and equipment, drugs, or supplies were provided by The University of Sydney. Ebrahim Bani Hassan reports financial support, administrative support, equipment, drugs, or supplies, and statistical analysis were provided by Australian Institute for Musculoskeletal Science (AIMSS).
Acknowledgements
This project was supported by the Australian Institute for Musculoskeletal Science (AIMSS) and a CIDP grant from the University of Sydney (ref. CIDP-1074). EBH held an Australian Medical Research Frontiers Fund Fellowship(MRFF: MACH-RART scheme 2019).
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