Effect of dexamethasone on the decrease of serum phenytoin concentrations

Clinically Significant Drug-Drug Interactions Involving Medications Used for Symptom Control in Patients With Advanced Malignant Disease: A Systematic Review

CONTEXT

Most patients with advanced malignant disease need to take several drugs to control symptoms. This treatment raises risks of serious adverse effects and drug-drug interactions (DDIs).

OBJECTIVES

To identify studies reporting clinically significant DDIs involving medications used for symptom control, other than opioids used for pain management, in adult patients with advanced malignant disease.

METHODS

Systematic review with searches in Embase, MEDLINE, and the Cochrane Central Register of Controlled Trials, from the start of the databases (Embase from 1980) through June 21, 2018. In addition, reference lists of relevant full-text articles were hand-searched.

RESULTS

Of 9699 retrieved citations, 462 were considered potentially eligible. After full-text reading, 29 were included in the final analysis, together with 13 articles from reference lists. The 42 included publications were case reports, letters to the Editor, and one retrospective study. Drugs most often involved were antiepileptics, antidepressants, corticosteroids, and nonopioid analgesics. Clinical manifestations of identified DDIs included sedation, respiratory depression, serotonin syndrome, neuroleptic malignant syndrome, delirium, seizures, ataxia, liver and kidney failure, bleeding, cardiac arrhythmias, rhabdomyolysis, and others. The most common mechanisms eliciting DDIs were alteration of CYP450-dependent metabolism and overstimulation of serotonin receptors in the central nervous system.

CONCLUSION

Drugs used for symptom control in patients with advanced cancer may cause serious DDIs. Although there is limited evidence for the risk of clinically significant DDIs, physicians treating patients with cancer should try to limit polypharmacy, avoid drug combinations with a high risk of DDIs, and closely monitor patients for adverse drug reactions.

Interactions of serum albumin, valproic acid and carbamazepine with the pharmacokinetics of phenytoin in cancer patients

Phenytoin dosing is critical in cancer patients as to decreased absorption secondary to chemotherapy-induced gastrointestinal toxicity, increased phenytoin metabolism in the liver secondary to chemotherapy, extreme patient profile that falls outside the predicted pharmacokinetic population, frequent hypoalbuminaemia and polydrug treatment. A retrospective study to assess the variability of free phenytoin and the free fraction of phenytoin, as well as the influence of comedication on these parameters was performed in cancer patients by using a population approach. Two hundred fifty-eight data pairs of total phenytoin and free phenytoin were analysed from 155 cancer patients on stable phenytoin using non-linear mixed-effect modeling (NONMEM). Total and free phenytoin were determined using a fluorescence polarization immunoassay. An extensive model building procedure was subsequently used for covariate testing on the free fraction of phenytoin. Mean total phenytoin concentration was 11.7 mg/l, free phenytoin 1.25 mg/l and phenytoin free fraction 0.107. Free phenytoin was <1 mg/l on 132 occasions (51.2%) and >2 mg/l on 37 occasions (14.3%). Total and free phenytoin were significantly correlated (r(S)=0.827, P<0.01). The free fraction of phenytoin was independent of time after drug intake. Serum albumin concentrations and comedication with valproic acid or carbamazepine were identified by NONMEM as significant determinants of phenytoin free fraction. Co-medication with valproic acid and carbamazepine led to a 52.5% and 38.5% increase of the free fraction of phenytoin, respectively, and a 10 g/l decrease of serum albumin to a 15.1% increase of the free fraction of phenytoin. Phenytoin pharmacokinetics could reliably be estimated from oral doses and steady-state concentrations of protein-bound and free phenytoin. The variability in the free fraction of phenytoin could partly be explained by the influence of albumin concentrations and antiepileptic comedication. Significant alterations of the free fraction of phenytoin and free phenytoin by co-administration of valproic acid or carbamazepine suggest therapeutic drug monitoring of free phenytoin to be of potential benefit in cancer patients.

Alterations in the distribution and orexigenic effects of dexamethasone in CAR-null mice

The constitutive androstane receptor (CAR, NR1I3) has emerged as an important regulator of drug metabolism. CAR responds to a wide spectrum of xenobiotics by inducing expression of cytochrome P450 (CYP) enzymes and a number of other proteins responsible for drug metabolism in the liver. The xenosensor function of CAR overlaps with that of the pregnane X receptor (PXR), another xenobiotic receptor that belongs to the nuclear hormone superfamily. We observed that injection of dexamethasone (Dex), a ligand for the glucocorticoid receptor (GR) and PXR but not CAR, results in an unexpected twofold increase in the stomach weight of CAR-null animals relative to wild-type animals. Here, we show that CAR knockout mice have elevated levels of Dex in the brain, resulting in a more rapid and robust increase in the hypothalamic expression of the GR-responsive target genes encoding neuropeptide Y (NPY) and neuropeptide Y receptor subtype 1 (NPY-R1). As expected, this is accompanied by a higher increase in the food intake of the CAR-null animals. The data described here highlight the complexity of the overlapping functions of CAR and PXR.

Possible interaction involving phenytoin, dexamethasone, and antineoplastic agents: a case report and review

OBJECTIVE

To describe a patient with subtherapeutic phenytoin concentrations and to review the literature regarding a possible interaction between phenytoin and chemotherapeutic agents as well as dexamethasone.

CASE SUMMARY

A 29-year-old white man with brain metastases secondary to malignant melanoma consistently had suboptimal phenytoin concentrations while receiving chemotherapy consisting of cisplatin, carmustine, dacarbazine, and tamoxifen. In addition, this patient received dexamethasone, which may have influenced his phenytoin concentrations.

DATA SOURCES AND STUDY SELECTION

Case reports were identified through a MEDLINE search and by cross referencing the articles identified.

DISCUSSION

The available literature addressing suboptimal phenytoin concentrations in the setting of chemotherapy is reviewed. Aggressive dosing of phenytoin may be required to achieve therapeutic concentrations in patients who are concurrently receiving chemotherapy and/or dexamethasone, especially in patients who fall outside the predictive pharmacokinetic model for phenytoin.

CONCLUSIONS

Subtherapeutic phenytoin concentrations may be decreased secondary to several proposed mechanisms: (1) the patient falls outside the predicted pharmacokinetic population parameters for phenytoin, (2) phenytoin absorption is decreased secondary to chemotherapy-induced gastrointestinal toxicity, and (3) metabolism of phenytoin is increased secondary to chemotherapy agents.