Derivatization of γ-Amino Butyric Acid Analogues for Their Determination in the Biological Samples and Pharmaceutical Preparations: A Comprehensive Review

γ-Aminobutyric acid (GABA) plays an important role in regulating neuronal excitability. Four structurally related drugs to GABA including pregabalin (PGB), gabapentin (GBP), vigabatrin (VGB), and baclofen are used for the treatment of central nervous system disorders. These drugs are small aliphatic molecules having neither fluorescent nor strong absorbance in the ultraviolet/visible region; therefore, direct determination of these analytes by optical methods is difficult. Additionally, their high boiling point makes gas chromatography impossible. Accordingly, the amine or acid moiety in these drugs is derivatized in order to improve their selectivity and sensitivity during determination in the biological samples. This review focuses on derivatization based methods and their different reactions for determination of PGB, GBP, VGB, and baclofen in the biological samples and pharmaceutical preparations reported between 1980 and 2020. High-performance liquid chromatography methods coupled with different detectors are a commonly used methods for determination of GABA analogs after derivatization. These methods cover 38.89% of all developed methods for determination of GABA analogs.

Use of high resolution/accurate mass full scan/data-dependent acquisition for targeted/non-targeted screening in equine doping control

High-resolution mass spectrometry (HRMS) is a very powerful technology for equine doping control analysis. The more recently developed hybrid type of Orbitrap-based HRMS instrument allows for both targeted and non-targeted screening analyses in a single liquid chromatography-high resolution tandem mass spectrometry (LC-HRMS/MS) run. In the present study, an LC-HRMS/MS method was developed and validated to detect prohibited substances in equine sports. The substances were recovered from equine plasma by liquid-liquid extraction (LLE) using methyl tert-butyl ether and were separated on a C18 reversed-phase column using mobile phases of 5 mM ammonium formate and acetonitrile. A 7.5 min LC gradient was employed to elute substances and results indicated that the LC method generated sharp and symmetric chromatographic peaks. An in-house equine doping compound database and a spectral library were built to increase method specificity for substances of interest. Five criteria, i.e. accurate mass, retention time, isotope pattern, selected HRMS/MS fragment ions (compound database) and HRMS/MS spectra (spectral library), were employed for targeted screening. We utilized these criteria to validate targeted detection of 451 substances within our in-house equine doping compound database. By using all five criteria in screening, the false screening positive rate is significantly reduced. A screening strategy and a Microsoft Excel macro were developed to facilitate interpretation and reporting of results. As the simultaneous acquisition of the full scan HRMS data provides the opportunity for retrospective non-targeted analysis, our findings highlight the use of this novel methodology as a simple, rapid, and reliably reproducible strategy to meet the challenge of identifying an increasing number of doping substances that could potentially impact the integrity of the horse racing community.

Doping detection in animals: A review of analytical methodologies published from 1990 to 2019

Despite the impressive innate physical abilities of horses, camels, greyhounds, or pigeons, doping agents might be administered to these animals to improve their performance. To control these illegal practices, anti-doping analytical methodologies have been developed. This review compiles the analytical methods that have been published for the detection of prohibited substances administered to animals involved in sports over 30 years. Relevant papers meeting the search criteria that discussed analytical methods aiming to detect and/or quantify doping substances in animal biological matrices published from 1990 to 2019 were considered. A total of 317 studies were included, of which 298 were related to horses, demonstrating significant advances toward the development of doping detection methods for equine sports. However, analytical methods for the detection of doping agents in sports involving other species are lacking. Due to enhanced accuracy and specificity, chromatographic analysis coupled to mass spectrometry detection is preferred over immunoassays. Regarding biological matrices, blood and urine remain the first choice, although alternative biological matrices, such as hair and feces, have been considered. With the increasing number and type of drugs used as doping agents, the analytes addressed in the published papers are diverse. It is very important to continue to detect and quantify these drugs, recognizing those that are most frequently used, in order to punish the abusers, protect animals' health, and ensure a healthier and genuine competition.

Investigation on gabapentin residues in eggs from free-range hens exposed to saline slags from pharmaceutical industry

A simple, sensitive and rapid liquid chromatography-tandem mass spectrometry method (LC-MS/MS) was developed and validated to monitor the presence of gabapentin as environmental contaminant in albumen and yolk of eggs from grazing flocks exposed to open air stored saline wastes from pharmaceutical industry. The method involved a simple liquid extraction followed by a gradient elution with formic acid 0.2 % and acetonitrile in reverse phase. ESI ionization was performed in positive ion mode. The tandem mass spectrometer was operated in multiple reaction monitoring mode. The calibration curves were linear in the concentration range from 5 to 400 ng/g for the two matrices with correlation coefficients that exceeded 0.990. The limits of quantitation were 12.0 and 14.8 ng/g in albumen and yolk, respectively. Results are discussed in light of the pharmacokinetics of gabapentin in experimentally exposed hens, accounting for the top soil intake in such free grazing animals.

Use of a soluble epoxide hydrolase inhibitor as an adjunctive analgesic in a horse with laminitis

HISTORY

A 4-year old, 500 kg Thoroughbred female horse diagnosed with bilateral forelimb laminitis and cellulitis on the left forelimb became severely painful and refractory to non-steroidal anti-inflammatory therapy (flunixin meglumine on days 1, 2, 3 and 4; and phenylbutazone on days 5, 6 and 7) alone or in combination with gabapentin (days 6 and 7).

PHYSICAL EXAMINATION

Pain scores assessed independently by three individuals with a visual analog scale (VAS; 0 = no pain and 10 = worst possible pain) were 8.5 on day 6, and it increased to 9.5 on day 7. Non-invasive blood pressure monitoring revealed severe hypertension.

MANAGEMENT

As euthanasia was being considered for humane reasons, a decision was made to add an experimental new drug, trans-4-{4-[3-(4-Trifluoromethoxy-phenyl)-ureido]-cyclohexyloxy}-benzoic acid (t-TUCB), which is a soluble epoxide hydrolase (sEH) inhibitor, to the treatment protocol. Dose and frequency of administration were selected based on the drug potency against equine sEH to produce plasma concentrations within the range of 30 nmol L(-1) and 2.5 μmol L(-1) . Pain scores decreased sharply and remarkably following t-TUCB administration and blood pressure progressively decreased to physiologic normal values. Plasma concentrations of t-TUCB, measured daily, were within the expected range, whereas phenylbutazone and gabapentin plasma levels were below the suggested efficacious concentrations.

FOLLOW UP

No adverse effects were detected on clinical and laboratory examinations during and after t-TUCB administration. No new episodes of laminitis have been noted up to the time of writing (120 days following treatment).

CONCLUSIONS

Inhibition of sEH with t-TUCB was associated with a significant improvement in pain scores in one horse with laminitis whose pain was refractory to the standard of care therapy. No adverse effects were noticed. Future studies evaluating the analgesic and protective effects of these compounds in painful inflammatory diseases in animals are warranted.

Effects of intraperitoneal administration of gabapentin on the minimum alveolar concentration of isoflurane in adult male rats

Gabapentin has been used to treat a variety of conditions in both human and veterinary medicine, including seizures, neuropathies and chronic pain. However, little information is known about the effects of gabapentin on the minimum alveolar concentration (MAC) of volatile anaesthetics. In this study, we investigated the effect of intraperitoneal administration of gabapentin on isoflurane MAC in adult male rats and hypothesized that gabapentin would decrease MAC in a dose-dependent manner. Using a standard MAC study protocol, we compared five treatment groups (G) receiving 0 (G(0)), 30 (G(30)), 100 (G(100)), 300 (G(300)) and 1000 (G(1000)) mg/kg gabapentin intraperitoneally and compared post-drug MAC values among groups and with corresponding baseline MAC values determined in each group prior to drug testing. The average baseline isoflurane MAC value was 1.45 ± 0.17%, which did not differ significantly between groups (1.47 ± 0.23% [G(30)], 1.46 ± 0.23% [G(100)], 1.48 ± 0.18% [G(300)] and 1.42 ± 0.2% [G(1000)]). In the G(300) and G(1000) groups, the isoflurane MAC value decreased significantly by 19% and 18%, respectively, from corresponding baseline values (P< 0.05, when compared with G(0)). Linear regression analysis revealed a negative correlation between blood gabapentin concentration and percent change in MAC (R(2) = 0.43; P< 0.05) but not dose. In conclusion, high-dose intraperitoneal gabapentin decreased isoflurane MAC. However, the effect was small and not dose-dependent, and is unlikely to be clinically significant.