Phenytoin metabolism to 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH) in man, cat and rat in vitro and in vivo, and susceptibility to phenytoin-induced gingival overgrowth

Developmental Toxicity and Biotransformation of Two Anti-Epileptics in Zebrafish Embryos and Early Larvae

The zebrafish (Danio rerio) embryo is gaining interest as a bridging tool between in-vitro and in-vivo developmental toxicity studies. However, cytochrome P450 (CYP)-mediated drug metabolism in this model is still under debate. Therefore, we investigated the potential of zebrafish embryos and larvae to bioactivate two known anti-epileptics, carbamazepine (CBZ) and phenytoin (PHE), to carbamazepine-10,11-epoxide (E-CBZ) and 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH), respectively. First, zebrafish were exposed to CBZ, PHE, E-CBZ and HPPH from 5¼- to 120-h post fertilization (hpf) and morphologically evaluated. Second, the formations of E-CBZ and HPPH were assessed in culture medium and in whole-embryo extracts at different time points by targeted LC-MS. Finally, E-CBZ and HPPH formation was also assessed in adult zebrafish liver microsomes and compared with those of human, rat, and rabbit. The present study showed teratogenic effects for CBZ and PHE, but not for E-CBZ and HPPH. No HPPH was detected during organogenesis and E-CBZ was only formed at the end of organogenesis. E-CBZ and HPPH formation was also very low-to-negligible in adult zebrafish compared with the mammalian species. As such, other metabolic pathways than those of mammals are involved in the bioactivation of CBZ and PHE, or, these anti-epileptics are teratogens and do not require bioactivation in the zebrafish.

Management of Generalized Gingival Enlargement in a Dog — Case Report and Literature Review

The present report describes treatment of a dog with generalized gingival enlargement and serves as a review of etiology and treatment options for this condition. The combined use of scalpel blade, electrosurgical equipment, and a 12-fluted bur on a high-speed handpiece with water irrigation allowed for accurate excision of excess gingival tissue and contouring of remaining gingiva. Surgery coupled with professional dental cleaning periodontal therapy can decrease the recurrence of this condition. In addition, home oral hygiene is an important component of the treatment plan. The history of patients with gingival enlargement should be reviewed carefully, including drug history and previous medical conditions, in an attempt to elucidate a specific cause and thus allow for greater long-term success.

Development and application of an ex vivo fosphenytoin nasal bioconversion/permeability evaluation method

There is an increasing interest in the intranasal delivery of central nervous system-active drugs due to the existence of a direct nose-to-brain connection. However, poor solubility limits the amount of drug that can be administered within an aqueous solution. In the present work, the objectives were to develop an ex vivo bioconversion/permeability evaluation method and to study the ex vivo bioconversion of the hydrophilic phosphate ester prodrug fosphenytoin (FOS) to the active drug phenytoin (PHT) and their comparative nasal permeation. Bioconversion/permeability studies were performed in excised porcine nasal mucosa mounted in Ussing chambers. The physical integrity of the tissues was evaluated by measurement of the transepithelial electrical resistance (TEER). The simultaneous quantitative assay of FOS, PHT and its major metabolite, 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH) was developed and validated according to international guidelines using a liquid chromatography analytical method. The FOS bioconversion rate and PHT and FOS apparent permeability coefficients (Papp) were determined at different time points. FOS bioconversion was also qualitatively investigated in human nasal mucus. The developed liquid chromatography method combines a fast and inexpensive sample preparation with inactivation of the enzymatic metabolism of the prodrug during sample manipulation and storage. It was linear, precise, accurate, and presented a high analyte recovery. FOS was converted ex vivo to PHT but the metabolite HPPH was not detected. The bioconversion rate increased with FOS concentration and with time, which suggests a diffusion-limited process. FOS was also converted to its active drug by human nasal mucus. A novel mathematical data analysis method was developed to reduce the bias introduced by variable mucosal TEER in the permeability results. At comparable FOS and PHT concentrations the ln(Papp(PHT)) of both compounds showed little difference, which indicates that the use of a hydrophilic and charged prodrug did not hinder overall drug permeation. At the highest tested FOS concentration it was possible to quantify FOS in the receiver chambers, meaning that at a sufficiently high concentration the FOS permeation rate overcame its bioconversion rate. The ln(Papp(PHT)) tended to similar equilibrium values as the assay progressed, but with higher FOS concentrations that equilibrium was attained faster. Acidic pH reduced the permeability of both PHT and FOS. The developed bioconversion/permeability evaluation method will constitute an important tool to select the most promising formulations before proceeding to in vivo studies. Importantly, it allowed the demonstration of phosphatase activity and FOS bioconversion in nasal mucosa, as well as the prodrug's nasal permeation potential. Furthermore, this study demonstrates the possibility of formulating phosphate prodrugs of poorly soluble central nervous system-active drugs as a strategy to increase the solubilized drug doses administered through the nasal route.

Feline orofacial pain syndrome (FOPS): a retrospective study of 113 cases

Feline orofacial pain syndrome (FOPS) is a pain disorder of cats with behavioural signs of oral discomfort and tongue mutilation. This report describes the findings from a case series of 113 cats including 100 Burmese. FOPS is suspected to be a neuropathic pain disorder and the predominance within the Burmese cat breed suggests an inherited disorder, possibly involving central and/or ganglion processing of sensory trigeminal information. The disease is characterised by an episodic, typically unilateral, discomfort with pain-free intervals. The discomfort is triggered, in many cases, by mouth movements. The disease is often recurrent and with time may become unremitting - 12% of cases in this series were euthanased as a consequence of the condition. Sensitisation of trigeminal nerve endings as a consequence of oral disease or tooth eruption appears to be an important factor in the aetiology - 63% of cases had a history of oral lesions and at least 16% experienced their first sign of discomfort during eruption of permanent teeth. External factors can also influence the disease as FOPS events could be directly linked to a situation causing anxiety in 20% of cats. FOPS can be resistant to traditional analgesics and in some cases successful management required anti-convulsants with an analgesic effect.

Characterization of cytochrome P450-mediated drug metabolism in cats

In this study we examined activities of cytochrome P450 (CYP)1A, 2C, 2D and 3A using hepatic microsomes from five male and five female cats. CYP1A, 2C, 2D and 3A activities were referred by ethoxyresorufin O-deethylation (EROD), tolbutamide hydroxylation (TBH), bufuralol 1'-hydroxylation (BLH) and midazolam 1'- and 4-hydroxylation respectively. The anti-rat CYP1A2 and CYP3A2 serum significantly inhibited EROD and midazolam 1'- and 4-hydroxylation, suggesting that EROD and midazolam 1'- and 4-hydroxylation were catalysed by CYP1A and 3A in cats respectively. Quinidine inhibited BLH in cats microsomes at quite low concentrations, suggesting that BLH was catalysed by CYP2D in cats. Tolbutamide hydroxylation activities were negligible in hepatic microsomes from both male and female cats, suggesting CYP2C activities of cats are extremely low. This suggests that CYP2C substrates should be carefully administered to cats. Although there is no sexual difference in CYP1A activities, there are differences in CYP2D and 3A activities of cats. CYP2D activities were higher (3-fold), but CYP3A activities were lower (one-fifth) in female cats. These results might suggest that CYP2D and 3A substrates should be prescribed for male and female cats using different dosage regimen.

Establishment of rat model of drug-induced gingival overgrowth induced by continuous administration of phenytoin

It is well-known that the anticonvulsant drug, phenytoin (PHT), induces gingival overgrowth as a side effect. The mechanism of PHT-induced gingival overgrowth, however, is not well understood. One reason for this is the lack of an adequate animal model for the PHT-induced gingival overgrowth. The purpose of this study was to establish a rat model of the drug-induced gingival overgrowth. Fourteen-day-old rats were randomly divided into 3 groups (5 rats/group). The control rats received only the vehicle. The rats in the experimental group were injected with 50 mg/kg per day (group L) and 100 mg/kg per day (group H) of PHT. They received a subcutaneous injection of vehicle or PHT twice a day for 42 days. A charge-coupled device (CCD) laser displacement sensor was used for measurement of the severity of gingival overgrowth of the mandibles. There was no significant difference in the growth of rats between the PHT-injected and the control groups. The CCD laser displacement sensor can measure minute changes in the gingival overgrowth in rats, and a significant extension of the buccal gingiva was observed in groups L and H. Using the CCD sensor, it is possible to quantify the change in the gingiva under precise control of the PHT dose.

Phenytoin and its metabolite, 5-(4-hydroxyphenyl)-5-phenylhydantoin, show bone resorption in cultured neonatal mouse calvaria

The effects of phenytoin and its major metabolite, 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH), on bone resorption of neonatal mouse calvaria were examined in vitro. Both phenytoin and HPPH induced significant bone resorption as compared to the controls after 72 h in culture. This effect may be the cause of phenytoin-induced bone loss in vivo.