Impact of gastrointestinal physiology on drug absorption in special populations--An UNGAP review

The release and absorption profile of an oral medication is influenced by the physicochemical properties of the drug and its formulation, as well as by the anatomy and physiology of the gastrointestinal (GI) tract. During drug development the bioavailability of a new drug is typically assessed in early clinical studies in a healthy adult population. However, many disease conditions are associated with an alteration of the anatomy and/or physiology of the GI tract. The same holds true for some subpopulations, such as paediatric or elderly patients, or populations with different ethnicity. The variation in GI tract conditions compared to healthy adults can directly affect the kinetics of drug absorption, and thus, safety and efficacy of an oral medication. This review provides an overview of GI tract properties in special populations compared to healthy adults and discusses how drug absorption is affected by these conditions. Particular focus is directed towards non-disease dependent conditions (age, sex, ethnicity, genetic factors, obesity, pregnancy), GI diseases (ulcerative colitis and Crohn's disease, celiac disease, cancer in the GI tract, Roux-en-Y gastric bypass, lactose intolerance, Helicobacter pylori infection, and infectious diseases of the GI tract), as well as systemic diseases that change the GI tract conditions (cystic fibrosis, diabetes, Parkinson's disease, HIV enteropathy, and critical illness). The current knowledge about GI conditions in special populations and their impact on drug absorption is still limited. Further research is required to improve confidence in pharmacokinetic predictions and dosing recommendations in the targeted patient population, and thus to ensure safe and effective drug therapies.

Potentially inappropriate prescribing in older primary care patients

OBJECTIVES

The aim of the study was to determine the rate of Potentially Inappropriate Medicines (PIM) and Potential Prescription Omissions (PPO) according to Screening Tool of Older Person's potentially inappropriate Prescriptions/Screening Tool to Alert doctors to the Right Treatment (STOPP/START) criteria.

STUDY DESIGN

A cross-sectional survey in community pharmacy.

METHOD

A prospective cross-sectional study was performed, during March-May 2012, in five community pharmacies. Patients aged ≥65 years, who collected one or more prescribed medications, were asked to participate in the study, and an interview was scheduled. Patients were asked to provide their complete medical and biochemical record from their general practitioner.

RESULTS

509 patients, mean age 74.8±6.5 years, 57.4% female, participated in the study. 164 PIM were identified in 139 patients (27.3%). The most common were: long-term use of long-acting benzodiazepines (20.7%), use of non-steroidal antiinflammatory drugs (NSAID) in patients with moderate-severe hypertension (20.1%), use of theophylline as monotherapy for chronic obstructive pulmonary disease (COPD, 15.9%) and use of aspirin without appropriate indication (15.2%). Patients with more than four prescpritions had a higher risk for PIM (OR 2.85, 95% CI 1.97-4.14, p<0.001). There were 439 PPO, identified in 257, (50.5%) patients. Predictors for PPO were older age, presence of diabetes, myocardial infarction, osteoporosis, stroke, COPD and/or angina pectoris.

CONCLUSION

STOPP/START criteria may be useful in identifying inappropriate prescribing and improving the current prescribing practices. Pharmacists should focus more on patients with more than four medications and/or patients with gout or pain accompanied with arterial hypertension because those patient may be at higher risk of PIM. Additionlly, patients older than 74 years with diabetes, osteoporosis, myocardial infarction, stroke, angina pectoris and/or COPD may have an increased risk of PPO.

The influence of drug-drug interaction and patients' characteristics on valproic acid's clearance in adults with epilepsy using nonlinear mixed effects modeling

Monitoring valproic acid (VPA) concentrations is especially challenging due to its highly variable pharmacokinetics (PK) and complex interactions with other antiepileptic drugs. We used sparse routine therapeutic drug monitoring data (n=200) from 129 adults with epilepsy to develop a population PK model of VPA, and determine the factors that influence its clearance (CL/F). Patients were on mono VPA therapy, or were concomitantly treated with carbamazepine, phenobarbital, topiramate (TPR), lamotrigine or benzodiazepines. A one-compartment model with first-order absorption and elimination was used to fit the concentration-time VPA data. Estimates generated by NONMEM indicated that VPA CL/F was influenced by the patients' body weight (increases with the 0.556 exponent), VPA daily dose (if it is greater than 1000 mg/day, CL/F increases by 43%), and co-therapy with TPR (lowering CL/F for 23%). The interindividual variability in VPA CL/F was modeled with exponentional error model. The estimated coefficient of variation was 31.9%, while the residual variability was 23.8% for the proportional and 13.2mg/l for the additive component. The model was validated in a separate set of 24 patients, and the predictive performance was evaluated, that indicated unbias and acceptable precision. This study confirms the interaction of VPA with TPR, which is presumably dependent on VPA dose.

Impact of age, weight and concomitant treatment on lamotrigine pharmacokinetics

WHAT IS KNOWN AND OBJECTIVE

Lamotrigine metabolism may be substantially altered with concomitant administration of valproic acid and/or carbamazepine. Such alterations may require the adjustment of lamotrigine dose to ensure optimal treatment efficacy and safety.

METHODS

The extent of lamotrigine interactions was investigated dependent on age, gender, weight and dose of concomitant carbamazepine and/or valproic acid in 65 patients with epilepsy. Lamotrigine plasma steady-state oral clearance (CLss/F) and area under the curve (AUCss) were calculated from the dose of drug, average steady-state concentration (Css) and interval of administration. Multiple regression analysis was used for the identification and quantification of factors that influenced lamotrigine pharmacokinetics.

RESULTS AND DISCUSSION

Age and dose of carbamazepine and valproic acid had significant influence on lamotrigine CLss/F and AUCss. Carbamazepine was associated with a dose-dependent increase and valproic acid with a dose-dependent decrease of lamotrigine metabolism rate. The effect of carbamazepine was more pronounced. Younger patients were expected to metabolize lamotrigine more rapidly whereas overweight patients may be less susceptible to interactions. Gender had no influence on lamotrigine pharmacokinetics.

WHAT IS NEW AND CONCLUSION

The efficacy and safety of lamotrigine may be altered by concomitant administration of carbamazepine and valproic acid. The models developed may be useful for estimating doses of lamotrigine for individual patients to minimize clinically significant interactions. Therapeutic monitoring is advisable when those drugs are used concomitantly.

Evaluation of drug-related problems in older polypharmacy primary care patients

AIMS AND OBJECTIVES

Targeting older patients with predictive factors for drug-related problems (DRPs) could make clinical medication reviews more cost-effective. The aim of this study was to identify the number, type, and potential predictive factors for DRPs in older polypharmacy patients.

METHODS

Community pharmacists performed clinical medication reviews and documented DRPs, types of interventions, and their implementation in older patients.

RESULTS

Three hundred eighty-eight medication reviews were analyzed, 964 DRPs (average 2.5 ± 1.9), and 1022 interventions (average 2.6 ± 2.0) were identified. The overall implementation rate of interventions was 70.1%, the highest was observed in interventions aiming to resolve the lack of therapy monitoring (86.8%). Patients with ≥12 medications had an increased risk of ≥5 DRPs (P < .001). Asthma was associated with lack of adherence (P = .002), lack of aspirin, statins, and proton pump inhibitors use with additional therapy needed (P = .002-.004). Predictive factors for drug interactions were antihypertensive medications and/or medications with narrow therapeutic index (P < .05). Lack of efficacy was associated with diabetes (P = .006). Nonsteroidal anti-inflammatory drugs were risk factors for inappropriate drug selection (P = .002). Lack of monitoring was associated with hypertension (P = .013), whereas benzodiazepines (P < .001) and aspirin (P = .021) were overused.

CONCLUSION

Patients with asthma, hypertension, and diabetes and lack of statin, antithrombotic agent, and/or proton pump inhibitor use were associated with higher risks for DRPs.

Association of Nrf2, SOD2 and GPX1 Polymorphisms with Biomarkers of Oxidative Distress and Survival in End-Stage Renal Disease Patients

The oxidative stress response via Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) interlinks inflammation- and metabolism-related pathways in chronic kidney disease. We assessed the association between polymorphisms in Nrf2, superoxide dismutase (SOD2), glutathione peroxidase (GPX1), and the risk of end-stage renal disease (ESRD). The modifying effect of these polymorphisms on both oxidative phenotype and ESRD prognosis, both independently and/or in combination with the glutathione S-transferase M1 (GSTM1) deletion polymorphism, was further analyzed. Polymorphisms in Nrf2 (rs6721961), SOD2 (rs4880), GPX1 (rs1050450), and GSTM1 were determined by PCR in 256 ESRD patients undergoing hemodialysis and 374 controls. Byproducts of oxidative stress were analyzed spectrophotometically or by ELISA. Time-to-event modeling was performed to evaluate overall survival and cardiovascular survival. The SOD2 Val/Val genotype increased ESRD risk (OR = 2.01, p = 0.002), which was even higher in combination with the GPX1 Leu/Leu genotype (OR = 3.27, p = 0.019). Polymorphism in SOD2 also showed an effect on oxidative phenotypes. Overall survival in ESRD patients was dependent on a combination of the Nrf2 (C/C) and GPX1 (Leu/Leu) genotypes in addition to a patients’ age and GSTM1 polymorphism. Similarly, the GPX1 (Leu/Leu) genotype contributed to longer cardiovascular survival. Conclusions: Our results show that SOD2, GPX1, and Nrf2 polymorphisms are associated with ESRD development and can predict survival.

Understanding variability in the pharmacokinetics of atypical antipsychotics - focus on clozapine, olanzapine and aripiprazole population models

Antipsychotic medicines are widely used for the management of psychotic symptoms regardless of the underlying diagnosis. Most atypical antipsychotics undergo extensive metabolism prior to excretion. Various factors may influence their pharmacokinetics, particularly elimination, leading to highly variable drug concentrations between individual patients following the same dosing regimen. Population pharmacokinetic approach, based on nonlinear mixed effects modeling, is a useful tool to identify covariates explaining pharmacokinetic variability, as well as to characterize and distinguish unexplained residual and between-subject (interindividual) variability. In addition, this approach allows the use of both sparsely and intensively sampled data. In this paper, we reviewed the pharmacokinetic characteristics of clozapine, olanzapine and aripiprazole, focusing on a population modeling approach. In particular, models based on a nonlinear mixed effects approach performed by NONMEM® software in order to identify and quantify sources of pharmacokinetic variability are presented. Population models were identified through systematic searches of PubMed and sixteen studies were selected. Some of the factors identified that significantly contribute to variability in elimination among clozapine, olanzapine, and aripiprazole are demographic characteristics, body weight, genetic polymorphism, smoking and in some cases drug interactions. Scientific research based on pharmacometric modeling is useful to further characterize sources of variability and their combined effect.

Pharmacokinetics of lamotrigine in paediatric and young adult epileptic patients--nonlinear mixed effects modelling approach

PURPOSE

The purpose of the study was to examine and describe adjunctive lamotrigine (LTG) pharmacokinetics in paediatric and young adult patients using a nonlinear mixed effects modelling (NONMEM) approach.

METHODS

The study included 53 patients (age range 3-35 years) who were concomitantly treated with carbamazepine (CBZ) and/or valproic acid (VPA). A total of 70 blood samples corresponding to trough levels were available for analysis. Data were modelled, and the final model was evaluated using NONMEM and auxiliary software tools.

RESULTS

The final LTG population model included the effects of concomitant drugs and patient's weight (WT) which stratified the population into three groups: ≤25 kg, >25 to <60 kg and ≥60 kg. Based on the final model, the estimated LTG oral clearance (CL/F) for a typical patient weighing ≤25 kg, >25 to <60 kg or ≥60 kg who was concomitantly treated with CBZ was estimated to be 3.28, 4.23, or 7.15 l/h, respectively. If a patient was concomitantly treated with CBZ + VPA, the CL/F decreased on average by 69.5 % relative to LTG + CBZ co-therapy. VPA was found to decrease the LTG CL/F by 87.6 % compared to co-therapy with only CBZ.

CONCLUSION

The LTG population pharmacokinetic model developed in this study may be a reliable method for individualising the LTG dosing regimen in paediatric and young adult patients on combination therapy during therapeutic drug monitoring.

Population pharmacokinetic model of Vancomycin based on therapeutic drug monitoring data in critically ill septic patients

PURPOSE

The present study aimed to establish a population pharmacokinetic model of vancomycin, including adult critically ill septic patients, with normal and impaired renal function.

MATERIALS AND METHODS

A prospective analysis of 146 concentrations from 73 adult critically ill septic patients treated with 1-h intravenous infusion of vancomycin were included in the study. A nonlinear mixed effects modeling (NONMEM) approach was applied for data analysis and evaluation of the final model. The influence of creatinine clearance calculated by the Cockcroft-Gault equation (CrCl), and other potential covariates on vancomycin clearance (CL) were evaluated.

RESULTS

The final one-compartment pharmacokinetic model includes the effect of CrCl on CL. Population pharmacokinetic values for a typical subject were estimated at 0.024 l/h for CL dependent on renal function (CLCrCl), 1.93 l/h for residual portion of CL (not dependent on renal function), and 0.511 l/kg for volume of distribution (V). According to the final model, for patients with CrCl = 120 ml/min, the median vancomycin total CL is 4.81 l/h, while CrCl-dependent fraction accounts for approximately 60% of CL.

CONCLUSIONS

The developed population vancomycin model may be used in estimating individual CL for adult critically ill septic patients, and could be applied for individualizing dosage regimens taking into account the continuous effect of CrCl.

Population pharmacokinetic model of carbamazepine derived from routine therapeutic drug monitoring data

The aim of the present study was to develop a population pharmacokinetic model of carbamazepine from routine therapeutic drug monitoring data. Steady-state carbamazepine plasma concentrations determined by homogenous enzyme immunoassay technique, dosing history including cotherapy, schedule of blood sampling, and patients' demographic characteristics were collected retrospectively from patients' chart histories. A one-compartment model was fitted to the data using nonlinear mixed effects modeling. The influence of weight, age, gender, smoking, allergy, carbamazepine daily dose, and cotherapy on clearance (CL/F) was evaluated. Additionally, bioavailability of controlled-release relative to immediate-release tablets was assessed. Two hundred sixty-five patients (423 concentrations) were used to develop a population pharmacokinetic model. The population estimate of CL/F from the base model was 5.14 L/h with interindividual variability of 50.20%. Patients' gender, age, smoking, allergy, cotherapy with lamotrigine and benzodiazepines had no effect on CL/F. Patient weight (WT), daily carbamazepine dose (DCBZ), daily dose of phenobarbitone (DPB) and valproic acid (VPA), when its daily dose exceeded 750 mg, significantly influenced CL/F and were included in the final model:[equation: see text] where VPA is 1 if dose is greater than 750 mg or 0 otherwise. No difference in bioavailability of carbamazepine between controlled- and immediate-release tablets was detected. The model predictions in the validation set had no bias and satisfactory precision. The model can be used for estimation of carbamazepine CL/F in individual patients in the postautoinduction phase and for selection of optimum dosing regimen in routine patient care.

Impact of community pharmacists' interventions on asthma self-management care

BACKGROUND

Asthma self-management is aimed to improve the quality and effectiveness of asthma care by supporting the patients to manage their illness by themselves.

OBJECTIVE

The aim of the study was to evaluate the impact of pharmacist-delivered counselling on patients knowledge and beliefs about the medicines, adherence level, and asthma control.

METHODS

A prospective intervention study was conducted in community pharmacies. A total of 90 patients completed the study. Four questionnaires were used: (1) Beliefs about medicines questionnaire (BMQ), (2) Knowledge of asthma and asthma medicine (KAM), (3) Asthma control test (ACT), and (4) 8-item Morisky medication adherence scale questionnaire (MMAS-8). Questionnaires were completed at baseline and 3 months later.

RESULTS

Low level of adherence and poor asthma control were determined initially. Better asthma control was significantly associated with higher adherence level, lower concerns regarding the medication use, and knowledge of triggers. Statistically significant improvement was found after 3 months in patients knowledge of asthma and its medications, their attitude towards medications (decrease in harm, overuse and concern; increase in necessity score), asthma control score (increased from 19 to 20, p < 0.05) and level of adherence (MMAS-8 score decreased from 3 to 2 p < 0.05). Better asthma control was achieved in 60% of patients. Sixteen patients (18%) were transferred from poor to well-controlled asthma, implying no need for patients' referral to the doctor and no additional cost for the health system.

CONCLUSIONS

Improved disease control could be a result of enhanced knowledge and understanding of the disease-medication relationship, improved inhalation technique, and support on patients' adherence. Acquired knowledge and skills, as well as improved attitude, empowered patients to take a more active part in asthma management. Education in further patients' follow-up should consider topics tailored to the patients' characteristics, needs, and prior counselling schedule with issues discussed.

Characterizing HIV-Preventive, Plasma Tenofovir Concentrations-A Pooled Participant-level Data Analysis From Human Immunodeficiency Virus Preexposure Prophylaxis Clinical Trials

BACKGROUND

Daily dosing of tenofovir disoproxil fumarate, with or without emtricitabine, has high efficacy in preventing human immunodeficiency virus (HIV) infection when individuals are adherent. The target protective plasma concentration of tenofovir (TFV), however, is not fully understood. The aim of this study is to estimate the protective TFV plasma concentration.

METHODS

Participant data from TFV-based daily oral and topical active arms of phase 3 trials (iPrEx, VOICE, and Partners PrEP) were pooled (n = 2950). Individual specific risk scores (low and high risk) of acquiring HIV, based on an earlier placebo analysis, were created. Longitudinal TFV pharmacokinetics (PK), HIV outcome, individual risk scores and the effect of sex at birth data were integrated and analyzed using non-linear mixed effects models.

RESULTS

Around 50% of the individuals were estimated to be adherent, which differed from self-reported adherence (∼90%) and large variation between longitudinal adherence patterns were identified. Following oral administration, the estimated protective TFV trough concentration was substantially higher in high-risk females (45.8 ng/mL) compared with high-risk males (16.1 ng/mL) and to low-risk individuals (∼7.5 ng/mL). Dosing simulations indicated that high-risk women require full adherence to maintain protective levels.

CONCLUSIONS

Using the largest PK-HIV outcome database to date, we developed a population adherence-PK-risk-outcome model. Our results indicate that high-risk females need higher levels of plasma TFV to achieve HIV protection compared with males. HIV protection exceeds 90% in all populations if daily adherence is achieved.

Predicting Optimal Dihydroartemisinin-Piperaquine Regimens to Prevent Malaria During Pregnancy for Human Immunodeficiency Virus-Infected Women Receiving Efavirenz

Background

A monthly treatment course of dihydroartemisinin-piperaquine (DHA-PQ) effectively prevents malaria during pregnancy. However, a drug-drug interaction pharmacokinetic (PK) study found that pregnant human immunodeficiency virus (HIV)-infected women receiving efavirenz-based antiretroviral therapy (ART) had markedly reduced piperaquine (PQ) exposure. This suggests the need for alternative DHA-PQ chemoprevention regimens in this population.

Methods

Eighty-three HIV-infected pregnant women who received monthly DHA-PQ and efavirenz contributed longitudinal PK and corrected QT interval (QTc) (n = 25) data. Population PK and PK-QTc models for PQ were developed to consider the benefits (protective PQ coverage) and risks (QTc prolongation) of alternative DHA-PQ chemoprevention regimens. Protective PQ coverage was defined as maintaining a concentration >10 ng/mL for >95% of the chemoprevention period.

Results

PQ clearance was 4540 L/day. With monthly DHA-PQ (2880 mg PQ), <1% of women achieved defined protective PQ coverage. Weekly (960 mg PQ) or low-dose daily (320 or 160 mg PQ) regimens achieved protective PQ coverage for 34% and >96% of women, respectively. All regimens were safe, with ≤2% of women predicted to have ≥30 msec QTc increase.

Conclusions

For HIV-infected pregnant women receiving efavirenz, low daily DHA-PQ dosing was predicted to improve protection against parasitemia and reduce risk of toxicity compared to monthly dosing.

Clinical Trials Registration

NCT02282293.

Application of Counter-propagation Artificial Neural Networks in Prediction of Topiramate Concentration in Patients with Epilepsy

PURPOSE

The application of artificial neural networks in the pharmaceutical sciences is broad, ranging from drug discovery to clinical pharmacy. In this study, we explored the applicability of counter-propagation artificial neural networks (CPANNs), combined with genetic algorithm (GA) for prediction of topiramate (TPM) serum levels based on identified factors important for its prediction.

METHODS

The study was performed on 118 TPM measurements obtained from 78 adult epileptic patients. Patients were on stable TPM dosing regimen for at least 7 days; therefore, steady-state was assumed. TPM serum concentration was determined by high performance liquid chromatography with fluorescence detection. The influence of demographic, biochemical parameters and therapy characteristics of the patients on TPM levels were tested. Data analysis was performed by CPANNs. GA was used for optimal CPANN parameters, variable selection and adjustment of relative importance.

RESULTS

Data for training included 88 measured TPM concentrations, while remaining were used for validation. Among all factors tested, TPM dose, renal function (eGFR) and carbamazepine dose significantly influenced TPM level and their relative importance were 0.7500, 0.2813, 0.0625, respectively. Relative error and root mean squared relative error (%) and their corresponding 95% confidence intervals for training set were 2.14 [(-2.41) - 6.70] and 21.5 [18.5 - 24.1]; and for test set were -6.21 [(-21.2) - 8.77] and 39.9 [31.7 - 46.7], respectively.

CONCLUSIONS

Statistical parameters showed acceptable predictive performance. Results indicate the feasibility of CPANNs combined with GA to predict TPM concentrations and to adjust relative importance of identified variability factors in population of adult epileptic patients.

High-performance liquid chromatographic method for the determination of moclobemide and its two major metabolites in human plasma

A selective, sensitive, and simple high-performance liquid chromatographic (HPLC) method was developed for the determination of moclobemide and its two major metabolites, Ro 12-5637 and Ro 12-8095, in human plasma. Sample preparation (0.5 ml of plasma) involved solid-phase extraction (SPE) using Speedisk H(2)O-Philic DVB columns. Separations were performed on a Waters XTerra RP18 column (5 microm, 150 mm x 4.6 mm). The mobile phase consisted of 10 mM KH(2)PO(4) with 1% triethylamine (pH 3.9) and acetonitrile (83:17, v/v), and a flow-rate was 1.2 ml/min. The total run time was 13 min. UV detection was performed at 240 nm. Mean absolute recoveries were > or =90% and the limit of quantification (LOQ) for all analytes was 0.02 mg/l. Calibration curves were linear (r>0.995) over a wide range of the analyte concentrations in plasma; thus, the method is suitable for different clinical studies when large variations in the drug/metabolites concentrations are observed. During a 5-day assay validation procedure the accuracy and precision were tested and proven (relative errors (RE)< or =13%; intra-day coefficient of variation (CV)< or =7%; inter-day CV< or =13%). Many drugs frequently used in the target patient population were evaluated for potential interference in order method selectivity to be ensured. The assay has been used in a clinical pharmacokinetic study to assess steady-state pharmacokinetics of moclobemide and two metabolites in depressive patients on mono- and combined therapy.

Population pharmacokinetics of topiramate in adult patients with epilepsy using nonlinear mixed effects modelling

The objective of the study was to develop population pharmacokinetic model of topiramate (TPM) using nonlinear mixed effects modelling approach. Data were collected from 78 adult epileptic patients on mono- or co-therapy of TPM and other antiepileptic drugs, such as carbamazepine (CBZ), valproic acid, lamotrigine, levetiracetam, phenobarbital and pregabalin. Steady-state TPM concentrations were determined in blood samples by high performance liquid chromatography with fluorescence detection. A one-compartment model with first order absorption and elimination was used to fit the concentration-time TPM data. Volume of distribution of TPM was estimated at 0.575 l/kg. The influence of demographic, biochemical parameters and therapy characteristics of the patients on oral clearance (CL/F) was evaluated. Daily carbamazepine dose (DCBZ) and renal function estimated by Modification of diet in renal disease (MDRD) formula significantly (p<0.001) influenced CL/F and were included in the final model: CL/F · (l/h)=1.53(l/h) · [1+0.476 · DCBZ(mg/day)/1000(mg/day)] · EXP[0.00476 · [MDRD(ml/ min)-95.72(ml/min)]]. Increase of CL/F with DCBZ and MDRD was best described by linear and exponential models. Mean TPM CL/F during CBZ co-therapy was 2.46 l/h, which is higher for 60.8% than in patients not co-treated with CBZ. Evaluation by bootstrapping showed that the final model was stable. The predictive performance was evaluated by adequate plots and indicated satisfactory precision. This model allows individualisation of TPM dosing in routine patient care, especially useful for patients on different CBZ dosing regimen.

Nonlinear mixed effects modelling approach in investigating phenobarbital pharmacokinetic interactions in epileptic patients

PURPOSE

The present study aimed to establish population pharmacokinetic model for phenobarbital (PB), examining and quantifying the magnitude of PB interactions with other antiepileptic drugs concomitantly used and to demonstrate its use for individualization of PB dosing regimen in adult epileptic patients.

METHODS

In total 205 PB concentrations were obtained during routine clinical monitoring of 136 adult epilepsy patients. PB steady state concentrations were measured by homogeneous enzyme immunoassay. Nonlinear mixed effects modelling (NONMEM) was applied for data analyses and evaluation of the final model.

RESULTS

According to the final population model, significant determinant of apparent PB clearance (CL/F) was daily dose of concomitantly given valproic acid (VPA). Typical value of PB CL/F for final model was estimated at 0.314 l/h. Based on the final model, co-therapy with usual VPA dose of 1000 mg/day, resulted in PB CL/F average decrease of about 25 %, while 2000 mg/day leads to an average 50 % decrease in PB CL/F.

CONCLUSIONS

Developed population PB model may be used in estimating individual CL/F for adult epileptic patients and could be applied for individualizing dosing regimen taking into account dose-dependent effect of concomitantly given VPA.

Effect of Long-term Topiramate Therapy on Serum Bicarbonate and Potassium Levels in Adult Epileptic Patients

BACKGROUND

Topiramate (TPM) is a sulfamate-substituted monosaccharide that is structurally different from other antiepileptic drugs. TPM inhibits carbonic anhydrase activity, which is associated with loss of bicarbonate from the kidney and consequently metabolic acidosis or electrolyte imbalance.

OBJECTIVE

The objectives of the study were to investigate the influence of TPM therapy on bicarbonate and potassium levels in adult epileptic patients.

METHODS

Data were collected from 59 adult patients on monotherapy or co-therapy of TPM and other antiepileptic drugs. Serum bicarbonate and potassium levels were available from all patients. Steady-state TPM trough concentrations were determined in blood samples by high-performance liquid chromatography. Data analysis was performed by SPSS software (version 17, Chicago, IL).

RESULTS

Patients were divided into group A (duration of therapy shorter than or equal to 5 years) and group B (duration of therapy longer than 5 years). Significant difference (P < 0.05) in serum bicarbonate levels was observed between these 2 groups. Bicarbonate levels were linearly related to the TPM therapy duration. No correlation was found between the TPM dose or patient age and bicarbonate or potassium levels, as well as between therapy duration and potassium level. Linear regression analysis showed no significant association among 54 available TPM trough concentrations and bicarbonate or potassium levels.

CONCLUSIONS

Results highlight the frequent occurrence of lower bicarbonate level associated with prolonged TPM therapy. Monitoring bicarbonate levels in patients on long-term TPM therapy might be useful.

Review of the Clinical Pharmacokinetics, Efficacy and Safety of Pembrolizumab

BACKGROUND

Treatment of various types of cancer has been improved significantly with the discovery of biological drugs that act as immune checkpoint inhibitors (ICIs). Pembrolizumab is a humanized monoclonal anti- PD-1 antibody currently approved for the treatment of a wide range of tumors, with more indications still being investigated in ongoing clinical trials.

OBJECTIVE

The aim of this paper is to present all currently available data regarding pembrolizumab pharmacokinetic and pharmacodynamic characteristics. Also, the possibility of using predictive biomarkers to monitor patients during cancer treatment is discussed.

METHODS

Database research was carried out (PubMed, ScienceDirect). Information was gathered from original articles, the European Medicines Agency datasheets and results from clinical trials.

RESULTS

This review summarizes present-day knowledge about the pharmacokinetics, different modeling approaches and dosage regimens, efficacy and safety of pembrolizumab and therapeutic monitoring of disease progression.

CONCLUSION

This review points out consistent pharmacokinetic characteristics of pembrolizumab in various cancer patients, the lack of pharmacokinetic-pharmacodynamic/outcome relationships, and the need for adequate biomarkers to predict treatment success. Hence, there is a clear necessity for more data and experience in order to optimize pembrolizumab treatment for each individual patient.

Exploring Sirolimus Pharmacokinetic Variability Using Data Available from the Routine Clinical Care of Renal Transplant Patients - Population Pharmacokinetic Approach

Background

Due to wide intra- and inter-individual pharmacokinetic variability and narrow therapeutic index of sirolimus, the therapeutic drug monitoring (TDM) of sirolimus with detailed biochemical and clinical monitoring is necessary for dose individualization in kidney transplant patients. The purpose of the study was to explore and identify factors that contribute to pharmacokinetic variability by developing and validating a population model using routine TDM data and routinely monitored biochemical and clinical parameters.

Methods

The data obtained by routine monitoring of 38 patients over a period of one year from the sirolimus treatment initiation, were collected from patients' records. Population analysis was performed using the software NONMEM^®. The validity of the model was tested by the internal and external validation techniques.

Results

The pharmacokinetic variability was partially explained with patient's age and liver function. CL/F was found to decrease with age. According to the developed model, sirolimus CL/F decreases by, in average, 37% in patients with aspartate aminotransferase (AST) greater than 37 IU/L. The internal and external validation confirmed the satisfactory prediction of the developed model.

Conclusions

The population modeling of routinely monitored data allowed quantification of the age and liver function influence on sirolimus CL/F. According to the final model, patients with compromised liver function expressed via AST values require careful monitoring and dosing adjustments. Proven good predictive performance makes this model a useful tool in everyday clinical practice.