Effect of saliva flow rate on saliva phenytoin concentrations: implications for therapeutic monitoring

Assessment of the toxic effects of levetiracetam on biochemical, functional, and redox parameters of salivary glands in male Wistar rats

Levetiracetam (LEV) is an anticonvulsant for epilepsy. The toxic effects of this medication in tissues have been associated with redox state imbalance, which can lead to salivary gland dysfunction. Therefore, the current work investigated the effects of LEV on the biochemical, functional, and redox parameters of the parotid and submandibular glands in rats. For this, male Wistar rats (Rattus norvegicus albinus) were randomly divided into 3 groups (n = 10/group): Control (0.9% saline solution), LEV100 (100 mg/kg), and LEV300 (300 mg/kg). After 21 consecutive days of intragastric gavage treatments, pilocarpine stimulated saliva secretion was collected for salivary biochemical analysis. The extracted salivary glands were utilized for histomorphometry and redox state analyses. Our results showed that LEV300 increased plasma hepatotoxicity markers and reduced salivary amylase activity and the acinar surface area of the parotid gland. Total oxidant capacity and oxidative damage to lipids and proteins were higher in the parotid gland, while total antioxidant capacity and uric acid levels were reduced in the submandibular gland of the LEV100 group compared to Control. On the other hand, total oxidant capacity, oxidative damage to lipids and proteins, total antioxidant capacity, and uric acid levels were lower in both salivary glands of the LEV300 group compared to Control. Superoxide dismutase and glutathione peroxidase activities were lower in the salivary glands of treated animals compared to Control. In conclusion our data suggest that treatment with LEV represents a potentially toxic agent, that contributes to drug-induced salivary gland dysfunction.

Saliva biomarkers in oral disease

Saliva is a versatile biofluid that contains a wide variety of biomarkers reflecting both physiologic and pathophysiologic states. Saliva collection is noninvasive and highly applicable for tests requiring serial sampling. Furthermore, advances in test accuracy, sensitivity and precision for saliva has improved diagnostic performance as well as the identification of novel markers especially in oral disease processes. These include dental caries, periodontitis, oral squamous cell carcinoma (OSCC) and Sjögren's syndrome (SS). Numerous growth factors, enzymes, interleukins and cytokines have been identified and are the subject of much research investigation. This review highlights current procedures for successful determination of saliva biomarkers including preanalytical factors associated with sampling, storage and pretreatment as well as subsequent analysis. Moreover, it provides an overview of the diagnostic applications of these salivary biomarkers in common oral diseases.

The Effect of Plasma Protein Binding on the Therapeutic Monitoring of Antiseizure Medications

Epilepsy is a widely diffused neurological disorder including a heterogeneous range of syndromes with different aetiology, severity and prognosis. Pharmacological treatments are based on the use, either in mono- or in polytherapy, of antiseizure medications (ASMs), which act at different synaptic levels, generally modifying the excitatory and/or inhibitory response through different action mechanisms. To reduce the risk of adverse effects and drug interactions, ASMs levels should be closely evaluated in biological fluids performing an appropriate Therapeutic Drug Monitoring (TDM). However, many decisions in TDM are based on the determination of the total drug concentration although measurement of the free fraction, which is not bound to plasma proteins, is becoming of ever-increasing importance since it correlates better with pharmacological and toxicological effects. Aim of this work has been to review methodological aspects concerning the evaluation of the free plasmatic fraction of some ASMs, focusing on the effect and the clinical significance that drug-protein binding has in the case of widely used drugs such as valproic acid, phenytoin, perampanel and carbamazepine. Although several validated methodologies are currently available which are effective in separating and quantifying the different forms of a drug, prospective validation studies are undoubtedly needed to better correlate, in real-world clinical contexts, pharmacokinetic monitoring to clinical outcomes.

Feasibility of Using Oral Fluid for Therapeutic Drug Monitoring of Antiepileptic Drugs

Therapeutic drug monitoring (TDM) of antiepileptic drugs (AED) using blood is well established but limited by its invasiveness, accessibility, cost, interpretation errors, and related disturbances in protein binding. TDM using oral fluid (OF) could overcome these limitations. This paper provides a summary of the current evidence for using OF as a matrix to perform TDM of AEDs, as well as practical considerations. A literature search of MEDLINE, EMBASE, and the Cochrane Library was conducted on April 9, 2018 (and then updated on May 20, 2020) using all AEDs as keywords along with "oral fluid," "saliva," "salivary," "seizure," "epilepsy," "antiepileptic," and "anticonvulsant." A total of 18 relevant articles were found and included in this review. There is evidence to suggest that AED TDM using OF is feasible and that reference ranges can be calculated for the following drugs: carbamazepine, ethosuximide, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, phenobarbital, phenytoin, primidone, topiramate, and valproic acid. For all other AEDs, there is either a lack of evidence on the feasibility of TDM using OF or the evidence indicates that TDM using OF is not feasible. Practical considerations should include the timing and method of OF collection (stimulated or unstimulated) due to their probable impact on the reliability of AED TDM. Using OF may improve the acceptability and accessibility and reduce the cost of AED TDM. Clinical implementation requires standardized collection protocols, more rigorously defined OF reference ranges, and further studies to determine the relevance to clinically important outcomes.

Salivary Monitoring of Antiepileptic Drugs

Therapeutic drug monitoring is widely used in the anticonvulsant treatment of persons with epilepsy. Most monitoring uses serum, but many anticonvulsant drugs can as easily be monitored using saliva, including phenobarbital, phenytoin, carbamazepine, lamotrigine, oxcarbazepine, topiramate, levetiracetam, and gabapentin. For highly protein-bound medications such as phenobarbital, phenytoin, and carbamazepine, saliva has the advantage of providing an approximation of the serum free level, the free level presumably being the active moiety. Salivary therapeutic drug monitoring offers a number of advantages over serum therapeutic drug monitoring, including lack of pain, lower cost, and wide potential acceptability by patients and physicians. It has the potential to open new approaches to treatment with strategic at-home monitoring at the time a seizure or adverse event occurs and to allow the collection of cohort-based, pharmacokinetic, and pharmcodynamic data for populations of persons of varying ages and with different medical conditions who require anticonvulsant medications.

Therapeutic drug monitoring of antiepileptic drugs by use of saliva

Blood (serum/plasma) antiepileptic drug (AED) therapeutic drug monitoring (TDM) has proven to be an invaluable surrogate marker for individualizing and optimizing the drug management of patients with epilepsy. Since 1989, there has been an exponential increase in AEDs with 23 currently licensed for clinical use, and recently, there has been renewed and extensive interest in the use of saliva as an alternative matrix for AED TDM. The advantages of saliva include the fact that for many AEDs it reflects the free (pharmacologically active) concentration in serum; it is readily sampled, can be sampled repetitively, and sampling is noninvasive; does not require the expertise of a phlebotomist; and is preferred by many patients, particularly children and the elderly. For each AED, this review summarizes the key pharmacokinetic characteristics relevant to the practice of TDM, discusses the use of other biological matrices with particular emphasis on saliva and the evidence that saliva concentration reflects those in serum. Also discussed are the indications for salivary AED TDM, the key factors to consider when saliva sampling is to be undertaken, and finally, a practical protocol is described so as to enable AED TDM to be applied optimally and effectively in the clinical setting. Overall, there is compelling evidence that salivary TDM can be usefully applied so as to optimize the treatment of epilepsy with carbamazepine, clobazam, ethosuximide, gabapentin, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, phenobarbital, phenytoin, primidone, topiramate, and zonisamide. Salivary TDM of valproic acid is probably not helpful, whereas for clonazepam, eslicarbazepine acetate, felbamate, pregabalin, retigabine, rufinamide, stiripentol, tiagabine, and vigabatrin, the data are sparse or nonexistent.

Non-invasive monitoring of phenytoin by reverse iontophoresis

Transdermal iontophoresis offers a non-invasive sampling method for therapeutic drug monitoring. This study examined whether iontophoretic extraction (a) is concentration dependent, (b) reflects the subdermal level of unbound drug, (c) follows protein binding changes, and (d) becomes truly non-invasive when a co-extracted compound is used as an internal standard for calibration. Iontophoresis was conducted in vitro using dermatomed pig-ear skin. The subdermal solution was a buffer containing phenytoin at therapeutic concentrations, an internal standard at fixed level, human albumin and/or valproic acid. The ionized form of phenytoin was recovered at the anode by electro-migration, while the neutral form was extracted to the cathode by electroosmosis. A satisfactory correlation between the reverse iontophoretic extracted amount of phenytoin and the subdermal concentration was observed. Iontophoresis extracted only the free fraction of phenytoin. At steady state, reverse iontophoresis monitored changes in free drug concentration provoked in the subdermal compartment. Acetate was introduced at a fixed concentration into the subdermal compartment to act as an 'internal standard'. Subsequently, acetate and the ionized form of phenytoin were co-extracted to the anode. The ratio of the extracted amounts was proportional to the subdermal concentration ratio demonstrating a means by which the method may become truly non-invasive.

The effect of multiple anticonvulsant therapy on the expression of phenytoin-induced gingival overgrowth

The potential effect of co-medication with phenobarbitone, primidone and carbamazepine on plasma and saliva concentrations of 5-(4-hydroxyphenyl)-5-phenylhydantoin (4-HPPH), the major metabolite of phenytoin in man and on the incidence of phenytoin-induced gingival overgrowth was investigated in a group of 36 adult epileptic patients. There were no significant differences in plasma or saliva concentrations of 4-HPPH or phenytoin in patients prescribed phenytoin alone, compared to those who received phenytoin with either phenobarbitone, primidone, or carbamazepine. In addition, the extent and the incidence of gingival overgrowth were similar in the 2 groups. The results suggest that chronic co-medication with other anti-convulsant drugs which induce phenytoin metabolism, does not affect the plasma or saliva 4-HPPB steady-state levels, nor the degree of gingival overgrowth in adult epileptic patients on therapy with phenytoin.

Therapeutic drug monitoring--antiepileptic drugs

AIMS

To provide a brief critical review of the basis and contemporary practice of monitoring the concentrations of antiepileptic drugs in biological fluids.

METHODS

The review is based on literature data and observations from clinical practice.

RESULTS

As experience has accumulated, monitoring of antiepileptic drug concentrations has come to be applied mainly to certain of the drugs when present in whole plasma. For these drugs there is a reasonably established relationship between drug concentrations and biological effects, but attention still needs to be paid to issues such as the timing of the measurements in relation to drug intake, the presence or absence of steady-state conditions, the presence in plasma of active metabolites and possible nonlinear pharmacokinetics of particular agents e.g. phenytoin.

CONCLUSIONS

Plasma antiepileptic drug concentration monitoring is coming to be used in a more thoughtful and critical manner. Lack of adequate knowledge of matters such as the relationship between the plasma concentrations and antiepileptic and toxic effects of the drugs, not only the newer, but also the longer established ones, in particular clinical situations, remains more important than deficiencies in analytical methodology in limiting the clinical usefulness of antiepileptic drug concentration monitoring.

Distribution kinetics of enoxacin and its metabolite oxoenoxacin in excretory fluids of healthy volunteers

The distribution kinetics of enoxacin and its main metabolite oxoenoxacin in excretory fluids was investigated in 11 healthy volunteers. A single intravenous dose of 428 mg of enoxacin was given as a 1-h infusion. Serial samples of plasma, urine, saliva, nasal secretions, tears, and sweat were drawn and analyzed for enoxacin and oxoenoxacin by reversed-phase high-pressure liquid chromatography. Large differences in the concentration-time profiles of the excretory fluids analyzed were observed. Nasal secretions exhibited the highest enoxacin exposure, as assessed by the area under the concentration-time curve. Excretory fluid/plasma area under the concentration-time curve ratios were found to be 1.67 +/- 0.36 for nasal secretions, 0.76 +/- 0.28 for saliva, 0.25 +/- 0.07 for sweat, and 0.23 +/- 0.11 for tears. The elimination half-life of enoxacin from sweat (8.27 +/- 2.63 h) was significantly longer than that for plasma (5.10 +/- 0.46 h). Oxoenoxacin was detected in urine and saliva and exhibited a higher renal clearance and a lower saliva exposure than the parent compound. In contrast to that of the metabolite, distribution of enoxacin in saliva was found to be time and pH dependent. In conclusion, our study revealed considerable differences in the distribution kinetics of enoxacin among various excretory sites. Because of distinct acidic and basic properties, the anionic oxometabolite significantly differs from the zwitterionic parent compound in its distribution characteristics.