Severe thrombocytopenia associated with phenytoin and cimetidine therapy

Drug-Drug Interactions Involving Dexamethasone in Clinical Practice: Myth or Reality?

Concomitant administration of multiple drugs frequently causes severe pharmacokinetic or pharmacodynamic drug-drug interactions (DDIs) resulting in the possibility of enhanced toxicity and/or treatment failure. The activity of cytochrome P450 (CYP) 3A4 and P-glycoprotein (P-gp), a drug efflux pump sharing localization and substrate affinities with CYP3A4, is a critical determinant of drug clearance, interindividual variability in drug disposition and clinical efficacy, and appears to be involved in the mechanism of numerous clinically relevant DDIs, including those involving dexamethasone. The recent increase in the use of high doses of dexamethasone during the COVID-19 pandemic have emphasized the need for better knowledge of the clinical significance of drug-drug interactions involving dexamethasone in the clinical setting. We therefore aimed to review the already published evidence for various DDIs involving dexamethasone in vitro in cell culture systems and in vivo in animal models and humans.

Use of Therapeutic Drug Monitoring, Electronic Health Record Data, and Pharmacokinetic Modeling to Determine the Therapeutic Index of Phenytoin and Lamotrigine

BACKGROUND

Defining a drug's therapeutic index (TI) is important for patient safety and regulating the development of generic drugs. For many drugs, the TI is unknown. A systematic approach was developed to characterize the TI of a drug using therapeutic drug monitoring and electronic health record (EHR) data with pharmacokinetic (PK) modeling. This approach was first tested on phenytoin, which has a known TI, and then applied to lamotrigine, which lacks a defined TI.

METHODS

Retrospective EHR data from patients in a tertiary hospital were used to develop phenytoin and lamotrigine population PK models and to identify adverse events (anemia, thrombocytopenia, and leukopenia) and efficacy outcomes (seizure-free). Phenytoin and lamotrigine concentrations were simulated for each day with an adverse event or seizure. Relationships between simulated concentrations and adverse events and efficacy outcomes were used to calculate the TI for phenytoin and lamotrigine.

RESULTS

For phenytoin, 93 patients with 270 total and 174 free concentrations were identified. A de novo 1-compartment PK model with Michaelis-Menten kinetics described the data well. Simulated average total and free concentrations of 10-15 and 1.0-1.5 mcg/mL were associated with both adverse events and efficacy in 50% of patients, resulting in a TI of 0.7-1.5. For lamotrigine, 45 patients with 53 concentrations were identified. A published 1-compartment model was adapted to characterize the PK data. No relationships between simulated lamotrigine concentrations and safety or efficacy endpoints were seen; therefore, the TI could not be calculated.

CONCLUSIONS

This approach correctly determined the TI of phenytoin but was unable to determine the TI of lamotrigine due to a limited sample size. The use of therapeutic drug monitoring and EHR data to aid in narrow TI drug classification is promising, but it requires an adequate sample size and accurate characterization of concentration-response relationships.

Drug-induced thrombocytopenia: a less known interaction

A 57-year-old woman with a left frontal lobe tumor was started on seizure prophylaxis with phenytoin and dexamethasone while awaiting elective surgery for tumor excision. Within a week, she developed a rash all over her body secondary to phenytoin hypersensitivity and her platelet counts decreased progressively to as low as 20,000/μl. Phenytoin was discontinued and she was given intravenous immunoglobulin for 2 days. She had progressive recovery of the platelet count to her baseline over the next 6 days. This case of phenytoin-induced thrombocytopenia emphasizes the importance of recognizing thrombocytopenia secondary to drug use and also highlights a lesser known but important interaction between phenytoin and dexamethasone to enhance this effect. We also provide a brief review of the literature and comment on the pathophysiology of this rare condition.

Possible acute thrombocytopenia post esomeprazole and hydantoin coadministration

Thrombocytopenia, defined as a platelet count less than 150 000/µL, occurs as a result of decreased production, sequestration, or peripheral destruction. Drug-induced thrombocytopenia is a clinically important adverse drug event involving many drugs including hydantoins. This report details an acute reaction of thrombocytopenia in a 55-year-old, critically ill, African American male patient after receiving a loading dose of fosphenytoin and a subsequent dose of IV phenytoin. The patient presented with an intracranial hemorrhage with hematoma and a blood pressure of 204/143 mm Hg. A fosphenytoin load infused for seizure prophylaxis and the first dose of a phenytoin maintenance regimen were followed by episodes of hypotension. In response to the hypotension, phenytoin was discontinued. On hospital day 2, the patient's platelet count had dropped dramatically from the morning before, 150 000 to 28 000/µL. The platelet count subsequently returned to baseline within 7 days of phenytoin discontinuation. The proposed cause of phenytoin-induced blood dyscrasias is direct or hapten-mediated toxicity by an arene oxide intermediate metabolite. Most documented cases of thrombocytopenia occur after a week or longer of phenytoin administration with the coadministration of glucocorticoids and cimetidine or proton pump inhibitors. An immediate decrease in platelets as seen in this case has not been previously described in the literature. Such a rapid induction of thrombocytopenia from phenytoin is suggestive of a direct cytotoxic effect on circulating platelets.

Myokymia and neuromyotonia in a cat

A 6-year-old spayed female domestic shorthair cat was examined because of a 2-week history of rhythmic muscle movements. Physical examination revealed thoracic limb rigidity, contracture of the carpi, generalized muscle atrophy, and rhythmic rippling of the muscles of all 4 limbs. Results of a CBC and serum biochemistry profile were unremarkable other than high creatine kinase activity. Electromyography revealed unique high-frequency discharges, including rhythmic bursts of single motor unit potentials appearing as doublets (myokymia) and more prolonged bursts of nonrhythmic motor unit potentials with characteristic waning amplitudes (neuromyotonia). Histologic examination of muscle biopsy specimens revealed noninflammatory necrotizing myopathy with regeneration. The cat did not respond to treatment with carbamazepine or prednisone but improved rapidly after treatment with phenytoin was initiated. Six months after initial examination, electromyography revealed a substantial decrease in the amount of spontaneous activity in previously affected muscles. However, the myokymic and neuromyotonic discharges were still present, albeit with a substantial decrease in frequency.

Drug-induced thrombocytopenia in the coronary care unit

Drug-induced thrombocytopenia is a phenomenon that causes significant morbidity and mortality among patients. Practitioners should be able to recognize the clinical manifestations of drug-induced thrombocytopenia, differentiate it from other causes, and manage it appropriately. Numerous case reports have documented drug-induced causes of thrombocytopenia. The following article focuses on the characteristics and management of drug-induced thrombocytopenia secondary to medications commonly encountered in the coronary care unit. Pharmacotherapeutic agents that are most commonly implicated in this setting include ticlopidine, unfractioned heparin, glycoprotein (GP)IIb/IIIa inhibitors, H(2)-receptor antagonists, quinidine and antibiotics. Case reports were obtained through a comprehensive search of the Medicine database and subsequently complemented by bibliographic reviews of the agents just specified. Reports that exhibited possible, probable, and definite associations with drug-induced thrombocytopenia are included in the article.

Phenytoin-induced thrombocytopenia

OBJECTIVE

To describe a patient with severe thrombocytopenia induced by the administration of phenytoin for prevention of seizures. A review of the literature supplements this case description to alert clinicians to this potentially serious hematologic reaction.

CASE SUMMARY

A woman who had experienced two seizures was prescribed phenytoin to prevent seizure recurrence. Further evaluation revealed a tumor, which was resected, and phenytoin was continued. Thrombocytopenia was noted 15 days after initiation of phenytoin, which was replaced with phenobarbital. Platelet transfusion and administration of intravenous immune globulin were used to treat her thrombocytopenia. Platelets were within the normal range by day 8 after the operation.

DISCUSSION

Phenytoin has been reported to induce various hematologic reactions, including thrombocytopenia. An intermediate epoxide metabolite of phenytoin is suspected as the cause of platelet destruction, which may occur via a complement-antibody reaction. Our patient experienced some confusion as a possible consequence of her thrombocytopenia, but no long-term sequelae followed.

CONCLUSIONS

Due to widespread use of phenytoin, clinicians must recognize the potential for the rare but serious adverse effect of thrombocytopenia, particularly in the neurosurgical population. Confusion, as observed in our patient, makes postoperative evaluation of central nervous system and cognitive function difficult, and can obscure the clinical presentation. At its worst extreme, disruption of platelet function may produce cerebral hemorrhage, which results in long-term functional deficits.

Thrombocytopenia following administration of phenytoin, dexamethasone and cimetidine: a case report and a potential mechanism

Cimetidine and phenytoin are useful medications often used together in patients with seizure disorders secondary to brain masses or metabolic abnormalities. We describe a case of thrombocytopenia in the setting of concurrent phenytoin, dexamethasone and cimetidine administration, and compare it with previously described cases of thrombocytopenia induced by concurrent use of phenytoin, cimetidine, and glucocorticoids. The similarities between these cases suggest mechanisms by which these agents may induce thrombocytopenia, specifically through potential downregulation of epoxide hydrolase by glucocorticoids.

Haematological adverse effects of histamine H2-receptor antagonists

Histamine H2-receptor antagonists are widely used in the treatment of gastrointestinal diseases related to gastric acid hypersecretion. Cimetidine was introduced into medical practice in 1976 and ranitidine, famotidine and nizatidine in 1981, 1985 and 1987, respectively. Haematological adverse effects are relatively uncommon and most have been reported in cases of cimetidine administration. These adverse effects are reviewed under 4 main headings: (a) blood cytopenias and leucocytosis; (b) coagulation disorders related to drug interactions with oral anticoagulants; (c) reduction of dietary iron absorption; and (d) reduction of dietary cobalamin absorption. 85 reported cases of blood cytopenias attributed to these drugs are reviewed, of which 75 (88%) were associated with cimetidine therapy. In postmarketing surveillance studies, the incidence of cimetidine-associated blood cytopenia has been evaluated at about 2.3 per 100,000 patients. Neutropenia and agranulocytosis are by far the most frequently encountered. Whatever the drug or the type of cytopenia, this adverse effect is almost always rapidly reversible when treatment is stopped. Moreover, in several cases other factors such as underlying diseases or additional drugs could have been responsible, at least partly, for the cytopenia. The pathophysiological basis of these adverse effects remains poorly explained. Various mechanisms have been proposed, which in some cases are probably associated: (a) direct toxicity for haemopoietic stem cells; (b) drug-induced immune reactions leading to blood or bone marrow cell damage, and (c) drug interactions, with increased and prolonged action of potentially haematotoxic drugs. Mechanisms (a) and (c) appear to be of particular clinical importance in cases of impaired renal elimination of H2-receptor antagonists. Cimetidine and probably to a lesser extent ranitidine potentiate the action of oral anticoagulants of both coumarin and indanedione structure. This may result in haemorrhagic complications. Such action is a consequence of the reduced hepatic metabolism of oral anticoagulants through a dose-dependent, reversible inhibition of cytochrome P450. Malabsorption of dietary iron and cobalamin appears to result from inhibition of gastric secretion by the H2-receptor antagonists. This is of no clinical importance in short term treatment, but long term use of H2-receptor antagonists may theoretically contribute to the occurrence of iron or cobalamin deficiency anaemia.

Phenytoin-induced thrombocytopenia

A 15-year-old boy developed thrombocytopenia and purpura two weeks after starting phenytoin therapy. The blood phenytoin level was in the toxic range. There was an increase in immature neutrophils but no abnormalities were present in other cell lines. Recovery was complete after drug therapy was discontinued. Thrombocytopenia is a rare isolated complication of phenytoin therapy. The probable autoimmune etiology distinguishes this syndrome from other phenytoin-induced blood dyscrasias.