Relevant pharmacological interactions between alkylating agents and antiepileptic drugs: Preclinical and clinical data.

Seizures are relatively common in cancer patients, and co-administration of chemotherapeutic and antiepileptic drugs (AEDs) is highly probable and necessary in many cases. Nonetheless, clinically relevant interactions between chemotherapeutic drugs and AEDs are rarely summarized and pharmacologically described. These interactions can cause insufficient tumor and seizure control or lead to unforeseen toxicity. This review focused on pharmacokinetic and pharmacodynamic interactions between alkylating agents and AEDs, helping readers to make a rational choice of treatment optimization, and thus improving patients' quality of life. As an example, phenobarbital, phenytoin, and carbamazepine, by increasing the hepatic metabolism of cyclophosphamide, ifosfamide and busulfan, yield smaller peak concentrations and a reduced area under the plasma concentration-time curve (AUC) of the prodrugs; alongside, the maximum concentration and AUC of their active products were increased with the possible onset of severe adverse drug reactions. On the other side, valproic acid, acting as histone deacetylase inhibitor, showed synergistic effects with temozolomide when tested in glioblastoma. The present review is aimed at providing evidence that may offer useful suggestions for rational pharmacological strategies in patients with seizures symptoms undertaking alkylating agents. Firstly, clinicians should avoid the use of enzyme-inducing AEDs in combination with alkylating agents and prefer the use of AEDs, such as levetiracetam, that have a low or no impact on hepatic metabolism. Secondly, a careful therapeutic drug monitoring of both alkylating agents and AEDs (and their active metabolites) is necessary to maintain therapeutic ranges and to avoid serious adverse reactions.

Population pharmacokinetic modelling of busulfan and the influence of body composition in paediatric Fanconi anaemia patients.

AIMS: Fanconi anaemia (FA) is a rare disorder characterized by progressive bone marrow failure that requires haematopoietic cell transplantation (HCT). Busulfan is used in conditioning regimens prior to HCT. Doses used in non-FA patients cause life-threatening toxicities in FA patients and data on busulfan pharmacokinetics (PK) in this population are limited. This study characterized busulfan PK in paediatric FA patients using population PK modelling and evaluated the effect of body composition on steady-state concentrations (Css ). METHODS: A total of 200 busulfan plasma concentrations in 29 FA patients from a recent study (Clinicaltrials.gov; NCT01082133) were available for population PK modelling. The effect of different body size-scaled doses and body compositions on Css was investigated using population PK modelling. RESULTS: Fat free mass (FFM) was identified as the best size descriptor in a two-compartment busulfan PK model in FA patients. Conventional dosing, based on an amount of busulfan per kilogram of total body mass, resulted in higher Css in FA patients with higher body mass index (BMI). A newly proposed FFM-based dosing strategy would eliminate the observed trend of higher concentrations in high BMI patients, and achieve consistent Css across a wide BMI spectrum. CONCLUSIONS: This is the first study to describe the population PK of busulfan in paediatric FA patients. The proposed model will facilitate PK model-informed precision dosing. FFM-based dosing is expected to improve the probability of achieving target Css , particularly in obese patients, while minimizing the risk of overdosing.

Systemic Absorption of Mitomycin-C When Used in Pterygium Surgery.

PURPOSE: To determine whether scleral topical application of mitomycin-C (MMC) results in measurable plasma levels of systemic absorption. METHODS: The study comprised 27 patients who were treated with MMC 0.2 mg/mL (0.02%) for 60 seconds during pterygium surgery. Blood samples were taken 30 minutes after surgery and evaluated by the human plasma liquid chromatography tandem-mass spectrometry method to determine the presence of MMC. RESULTS: The amount of MMC in 27 samples tested was determined as below 0.25 ppb (ng/mL). CONCLUSIONS: In this study of 27 patients treated with topical application of MMC for pterygium surgery, there was no measurable evidence of systemic drug absorption. Although systemic absorption has been found with the use of larger quantities of MMC, there is an extremely low likelihood of systemic absorption or toxicity of MMC after pterygium surgery.

Therapeutic Drug Monitoring of Busulfan for the Management of Pediatric Patients: Cross-Validation of Methods and Long-Term Performance.

BACKGROUND: Busulfan (Bu) is an alkylating agent used as part of the conditioning regimen in pediatric patients before hematopoietic stem cell transplantation. Despite intravenous (IV) administration and dosing recommendations based on age and weight, reports have revealed interindividual variability in Bu pharmacokinetics and the outcomes of hematopoietic stem cell transplantation. In this context, adjusting doses to Bu's narrow therapeutic window is advised. We aimed to assess the utility of therapeutic drug monitoring (TDM) of Bu in children, the reliability of Bu quantification methods, and its stability in plasma when stored for up to 5 years. METHODS: Eighteen patients from our TDM center (252 samples) were included. All of them received a 2-hour Bu IV infusion 4 times daily for a total of 16 doses. The first dose of Bu was age/weight-based, and the subsequent doses were adjusted from third or fifth dose onward based on the estimated first dose pharmacokinetic parameters to target steady-state concentrations (Css) of 600-900 ng/mL. The performance of our unit's high-performance liquid chromatography with tandem mass spectrometry method was assessed using a quality control (QC, 35 series) chart. International, multicenter, cross-validation test (n = 21) was conducted to validate different analytical methods. To assess Bu stability, regression analyses and Bland-Altman plots were performed on measurements at repeated time points on samples stored at -80°C for up to 5 years. RESULTS: We observed a 4.2-fold interindividual variability in Bu Css after the first dose, with only 28% of children having a Css within the target range. During the 4 days of conditioning, 83% of children had their doses modified according to TDM recommendations. This achieved a Css within the target range in 75% of the children. Routine QC measurements were generally within the ±15% range around theoretical values, showing the optimal robustness of our center's analytical method. Two of the 21 Bu TDM centers returned inadequate results during cross-validation testing; both used a UV detection method. Storage at -80°C led to a fall in Bu content of 14.9% ± 13.4% at 2-4 years and of 20% ± 5% by 5 years (roverall = 0.92). CONCLUSIONS: We conclude that TDM is an effective method of achieving targeted Bu levels in children. QC programs are crucial to monitoring and maintaining the quality of an analytical method.

Pharmacokinetics of metronomic chemotherapy: a neglected but crucial aspect.

Metronomic chemotherapy describes the close, regular administration of chemotherapy drugs at less-toxic doses over prolonged periods of time. In 2015, the results of randomized phase III clinical trials demonstrated encouraging, albeit limited, efficacy benefits of metronomic chemotherapy regimens administered as adjuvant maintenance therapy for the treatment of breast cancer, or as maintenance therapy in combination with an antiangiogenic agent for metastatic colorectal cancer. Owing to the investigational nature of this approach, metronomic chemotherapy regimens are highly empirical in terms of the optimal dose and schedule for the drugs administered; therefore, greater knowledge of the pharmacokinetics of metronomic chemotherapy is critical to the future success of this treatment strategy. Unfortunately, such preclinical and clinical pharmacokinetic studies are rare. Herein, we present situations in which active drug concentrations have been achieved with metronomic schedules, and discuss their associated pharmacokinetic parameters. We summarize examples from the limited number of clinical studies in order to illustrate the importance of assessing such pharmacokinetic parameters, and discuss the influence this information can have on improving efficacy and reducing toxicity.

High-Throughput Validated Method for the Quantitation of Busulfan in Plasma Using Ultrafast SPE-MS/MS.

BACKGROUND: Busulfan is an alkylating agent used to ablate bone marrow cells before hematopoietic stem cell transplantation. Because of its highly variable pharmacokinetics, studies have shown that therapeutic drug monitoring is clinically useful for patients undergoing bone marrow transplant so that toxic effects associated with high drug exposure could be reduced and improve clinical outcomes. Current methods for assaying busulfan include the use of gas chromatography mass spectrometry (GC/MS), high-performance liquid chromatography, and liquid chromatography mass spectrometry. The clinical need for faster turnaround times and increased testing volumes has required laboratories to develop faster methods of analysis for higher throughput of samples. Therefore, we present a method for the quantification of busulfan in plasma using an ultrafast solid-phase extraction/tandem mass spectrometry, which has much faster sample cycle times and similar analytical results to GC/MS. METHOD: Calibration standards, quality controls, and patient samples after addition of busulfan-d4 internal standard were extracted into n-butyl chloride from plasma. The organic fraction was dried and reconstituted in 600 µL of water containing ammonium acetate, trifluoroacetic acid, and formic acid. Sample analysis was performed at a rate of less than 20 seconds per sample using a Rapidfire 300 system coupled to an Agilent 6490 MS/MS using electrospray ionization in positive ion mode. Concentrations were calculated based on a 5-point calibration curve using a 1/x linear curve fit. RESULTS: The analytical method shows excellent precision, sensitivity, and specificity. Minimal ion suppression or enhancement due to the matrix effect was observed. No significant carryover was seen following a sample containing 15,000 ng/mL of busulfan. Seventy-two patient samples were cross-validated with a current GC/MS method. All patient results throughout the analytical range correlated within the acceptance criteria of ±20%. The linear regression demonstrated the following: slope = 1.0067, r = 0.9964, and intercept = -6.2. CONCLUSIONS: A simple, fast, and robust method was developed for the quantitation of busulfan in plasma with solid-phase extraction/tandem mass spectrometry cycle times of <20 seconds per sample.

Significance of tagI and mfd genes in the virulence of non-typeable Haemophilus influenzae

Non-typeable Haemophilus influenzae (NTHi) is an opportunist pathogen well adapted to the human upper respiratory tract and responsible for many respiratory diseases. In the human airway, NTHi is exposed to pollutants, such as alkylating agents, that damage its DNA. In this study, we examined the significance of genes involved in the repair of DNA alkylation damage in NTHi virulence. Two knockout mutants, tagI and mfd, encoding N3 methyladenine-DNA glycosylase I and the key protein involved in transcription-coupled repair, respectively, were constructed and their virulence in a BALB/c mice model was examined. This work shows that N3 -methyladenine-DNA glycosylase I is constitutively expressed in NTHi and that it is relevant for its virulence (AU)

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Protective effect of ethanolic extract of Tabernaemontana divaricata (L.) R. Br. against DEN and Fe NTA induced liver necrosis in Wistar Albino rats.

This study is an attempt to evaluate the hepatoprotective activity of Tabernaemontana divaricata against DEN and Fe NTA induced liver necrosis in rats. Ethanolic extract of the whole plant of Tabernaemontana divaricata at doses of 200 and 400 mg/kg body weight and 5-fluorouracil (standard drug) was orally administered to male Wistar Albino rats once daily for 24 weeks, simultaneously treated with the carcinogen DEN and Fe NTA. In simultaneously treated animals, the plant extract significantly decreased the levels of uric acid, bilirubin, AST, ALT, and ALP in serum and increased the levels of liver marker enzymes in liver. Treatment with the extracts resulted in a significant increase in the levels of antioxidants accompanied by a marked reduction in the levels of malondialdehyde when compared to DEN and Fe NTA treated group. When compared with 200 mg/kg bw rats, 400 mg/kg bw rats and 5-fluorouracil treated rats showed better results in all the parameters. The histopathological studies confirmed the protective effects of extract against DEN and Fe NTA induced liver necrosis. Thus, it could be concluded that the use of Tabernaemontana divaricata extract in the treatment of carcinogen induced hepatic necrosis.

Predictive performance of a physiologically based pharmacokinetic model of busulfan in children.

A physiologically based pharmacokinetic (PBPK) model of the DNA-alkylating agent busulfan was slightly modified and scaled from adults to children in order to predict the systemic busulfan drug exposure in children. Capitalizing on the recent major software release of PK-Sim®, we refined our PBPK model by implementing glutathione S transferase (GST) in 11 organs using the software integrated enzyme expression database. In addition, two irreversible binding processes (i.e., DNA and plasma protein binding) were applied by using Koff and KD values. The model was scaled from adults to children. Simulations were computed and compared to concentration-time data after intravenous (i.v.) busulfan administration to 36 children. Based on the results, an age-dependent enzyme activity and maturation ratio was tailored and evaluated with an external dataset consisting of 23 children. Initial adult to children scaling indicated lower clearance values for children in comparison to adults. Subsequent age-dependent maturation ratio resulted in three different age groups: Activity of busulfan-glutathione conjugate formation was 80%, 61%, and 89% in comparison to adults for children with an age of up to 2 years, > 2-6 years, and > 6-18 years, respectively. Patients of the evaluation dataset were simulated with a mean percentage error (MPE) for all patients of 3.9% with 3/23 children demonstrating a MPE of > ±30%. The PBPK model parameterization sufficiently described the observed concentration-time data of the validation dataset while showing an adequate predictive performance. This PBPK model could be helpful to determine the first dose of busulfan in children.

Pharmacogenetic aspects of drug metabolizing enzymes in busulfan based conditioning prior to allogenic hematopoietic stem cell transplantation in children.

Allogenic hematopoietic stem cell transplantation (HSCT) is a well established but complex treatment option for malignant and non-malignant disorders in pediatric patients. Most commonly used myeloablative and non-myeloablative conditioning regimens in children comprise alkylating agents, such as busulfan (BU) and cyclophosphamide. Inter-individual variability in the pharmacokinetics of BU can result in altered conditioning of the patient and therefore lead to relapse or rejection due to under exposures, or occurrence of toxicities due to over exposures. With the introduction of the intravenous formulation of BU, this variability has been reduced but still cannot be fully predicted. Inter and intra-individual variability of BU kinetics is more common in children compared to adults and toxicity of BU based regimens is still a concern. It has been hypothesized that some of this variability in BU pharmacokinetics and treatment outcomes, especially the toxicity, might be predicted by genetic variants of enzymes involved in the metabolism of BU. This review intends to summarize the studies performed to date on the pharmacokinetics and pharmacogenetics of BU based conditioning, specifically in relation to children.

Towards evidence-based dosing regimens in children on the basis of population pharmacokinetic pharmacodynamic modelling.

When growing up, the pharmacokinetic (PK) and pharmacodynamic (PD) profiles of drugs change, which may alter the effect of drugs. To ensure optimal drug efficacy and safety in paediatric care, PK and PD relationships of drugs need to be explored in children. This article presents an outline on performing a population PK/PD study and translating these results into rational dosing regimens, with the development and prospective evaluation of PK/PD derived evidence-based dosing regimen being discussed. Examples on amikacin, morphine and busulfan are provided, showing how PK(/PD) modelling not only led to optimization and individualization in paediatric clinical care for the specific drugs but also to insight in maturation of organ systems involved. It is shown that the latter results can subsequently be used as a basis for dosing of other drugs eliminated through the same pathway. Ultimately, these efforts should lead to predictable drug efficacy and safety across all age groups.

Influence of fludarabine on the pharmacokinetics of oral busulfan during pretransplant conditioning for hematopoietic stem cell transplantation.

This study evaluated the influence of fludarabine on the pharmacokinetics of busulfan administered orally to patients receiving a conditioning regimen for hematopoietic allogeneic stem cell transplantation (HSCT). Twenty-six patients treated with oral busulfan (1 mg/kg/6 h for 4 days) were divided into two groups according to the concomitant administration of fludarabine (n = 11; 30 mg/m(2) for 5 days) or subsequent administration of cyclophosphamide (n = 15; 60 mg/kg for 2 days). Serial blood samples were collected on Day 4 of busulfan administration. Plasma busulfan concentrations were determined by HPLC-UV and the pharmacokinetic parameters were calculated using the WinNonlin program. Patients concomitantly treated with fludarabine showed reduced apparent clearance of busulfan (110.5 mL/h/kg vs. 157.4 mL/h/kg) and higher AUC0-6 (area under the plasma concentrations vs. time curve) than patients subsequently treated with cyclophosphamide (7.9 µg h/mL vs. 5.7 µg h/mL). No association was observed between busulfan AUC0-6 and clinical evolution of the patients. Although plasma busulfan concentrations were higher in patients receiving concomitant fludarabine, myelosuppression-related toxicity was less frequent than in patients treated with busulfan and cyclophosphamide. The results suggest that patients treated with fludarabine should receive 30% lower busulfan doses during conditioning protocols for HSCT.

Variation in prescribing patterns and therapeutic drug monitoring of intravenous busulfan in pediatric hematopoietic cell transplant recipients.

Personalizing intravenous (IV) busulfan doses in children using therapeutic drug monitoring (TDM) is an integral component of hematopoietic cell transplant. The authors sought to characterize initial dosing and TDM of IV busulfan, along with factors associated with busulfan clearance, in 729 children who underwent busulfan TDM from December 2005 to December 2008. The initial IV busulfan dose in children weighing ≤12 kg ranged 4.8-fold, with only 19% prescribed the package insert dose of 1.1 mg/kg. In those children weighing >12 kg, the initial dose ranged 5.4-fold, and 79% were prescribed the package insert dose. The initial busulfan dose achieved the target exposure in only 24.3% of children. A wide range of busulfan exposures were targeted for children with the same disease (eg, 39 target busulfan exposures for the 264 children diagnosed with acute myeloid leukemia). Considerable heterogeneity exists regarding when TDM is conducted and the number of pharmacokinetic samples obtained. Busulfan clearance varied by age and dosing frequency but not by underlying disease. The authors- group is currently evaluating how using population pharmacokinetics to optimize initial busulfan dose and TDM (eg, limited sampling schedule in conjunction with maximum a posteriori Bayesian estimation) may affect clinical outcomes in children.

Systemic absorption of mitomycin-C when used in refractive surgery.

PURPOSE: To determine whether corneal topical application of mitomycin-C (MMC) results in measurable plasma levels of systemic absorption. SETTING: Madigan Army Medical Center, Refractive Surgery Center, Fort Lewis, Washington, and Micro-Constants Laboratory, San Diego, California, USA. DESIGN: Case-control study. METHODS: The study comprised male and female active-duty soldiers having excimer laser photorefractive keratectomy with MMC. Patients who met inclusion criteria were asked to provide a blood sample immediately after being treated with MMC 0.2 mg/mL (0.02%) for 30 seconds. Human plasma samples were evaluated by liquid chromatography mass spectrometry to determine whether MMC was present. RESULTS: Thirty samples were submitted for evaluation. There was zero detection of MMC in the submitted samples. The quantifiable limit was greater than 10.0 ng/mL. All samples were below this. CONCLUSIONS: In this study of 30 patients with topical application of MMC for refractive surgery, there was no measurable evidence of systemic absorption. Although systemic absorption has been found with use in larger quantities, it was not known whether MMC toxicity concerns could be extrapolated to the refractive surgery population. This information allows counseling of patients on the extremely low likelihood of systemic absorption or toxicity following current techniques for refractive surgery. FINANCIAL DISCLOSURE: No author has a financial or proprietary interest in any material or method mentioned.

A method for the simultaneous determination of mercapturic acids as biomarkers of exposure to 2-chloroprene and epichlorohydrin in human urine.

We developed and validated an analytical method for the simultaneous determination of several chlorine and non-chlorine containing mercapturic acids in urine as specific metabolites of the hazardous chemicals 2-chloroprene and epichlorohydrin. The method involves an online column switching arrangement for online solid phase extraction of the analytes with subsequent analytical separation and detection using LC-MS/MS. The developed method enables for the first time the determination of Cl-MA-I (4-chloro-3-oxobutyl mercapturic acid), Cl-MA-II (4-chloro-3-hydroxybutyl mercapturic acid), Cl-MA-III (3-chloro-2-hydroxy-3-butenyl mercapturic acid) and HOBMA (4-hydroxy-3-oxobutyl mercapturic acid) as potential biomarkers of 2-chloroprene in urine. Additionally, CHPMA (3-chloro-2-hydroxypropyl mercapturic acid) as a specific metabolite of epichlorohydrin in urine and DHBMA (3,4-dihydroxybutyl mercapturic acid) can be determined. The analytical method proved to be both sensitive and reliable with detection limits ranging from 1.4 µg/L (for Cl-MA-III) to 4.2 µg/L (for HOBMA). Intra- and interday imprecision was determined to range from 4.7 to 11.8%. Due to the good accuracy and precision and the low limits of detection the developed method is well suited for application in biomonitoring studies in order to determine occupational exposure to 2-chloroprene and epichlorohydrin.

Influence of GST gene polymorphisms on the clearance of intravenous busulfan in adult patients undergoing hematopoietic cell transplantation.

Intravenous (i.v.) busulfan can produce a more consistent pharmacokinetic profile than oral formulations can, but nonetheless, significant interpatient variability is evident. We investigated the influence of polymorphisms of 3 GST isozyme genes (GSTA1, GSTM1, and GSTT1) on i.v. busulfan clearance. Fifty-eight adult patients who received 3.2 mg/kg/day of busulfan as conditioning for hematopoietic cell transplantation were included in this study. Stepwise multiple linear regression demonstrated that GSTA1 variant GSTA1∗B (P = .004), GSTM1/GSTT1 double-null genotype (P = .039), and actual body weight (P = .001) were significantly associated with lower clearance of i.v. busulfan. A trend test analyzing the overall effect of GST genotype on busulfan pharmacokinetics, combining GSTA1 gene polymorphism and the number of GSTM1- and GSTT-null genotypes, showed a significant correlation between GST genotype and busulfan clearance (P = .001). The clearance of i.v. busulfan was similar between patients with GSTA1∗A/∗A and GSTM1/GSTT1 double-null genotypes and those with GSTA1∗A/∗B and GSTM1/GSTT1 double-positive genotypes. In conclusion, a pharmacogenetic approach using GST gene polymorphisms may be valuable in optimizing the i.v. busulfan dosage scheme. Our results also highlight the importance of including polygenic analyses and addressing interactions among isozyme genes in pharmacogenetic studies.

Influence on Busilvex pharmacokinetics of clonazepam compared to previous phenytoin historical data.

This study investigated the effect of seizure prophylaxis on busulfan (Bu) plasma exposure. Twenty-four adult patients received an intravenous Bu-cyclophoshamide conditioning regimen prior to bone marrow transplantation. Busilvex (0.8 mg/kg) was administered every six hours during four consecutive days. Clonazepam (0.025 to 0.03 mg/kg/day as a continuous 12-h i.v. infusion) was administered at least 12 hours prior to i.v. Bu dosing and continued until 24 hours after the last dose. Pharmacokinetic (PK) data were compared with those previously collected in patients (n=127) treated with phenytoin for seizure prophylaxis. Through population PK analysis, a 10% average increase (coefficient of variation, RSE=5.35%) in total clearance of Bu was quantified when Bu was associated with clonazepam as compared to phenytoin, which was considered as not being clinically relevant. The suspected induction on Bu metabolism by phenytoin should have resulted in the opposite effect. The patient efficacy and safety profiles were comparable between the two cohorts.

Busulfan systemic exposure after oral administration of extemporeanously prepared high-dose busulfan capsules.

PURPOSE: The aim of the study was to analyze patients' busulfan (BU) exposure after oral administration of extemporeanously prepared BU capsules prior to blood stem cell transplantation. METHODS: Patients were treated with 1 mg/kg body weight BU administered orally every 6h on each of 4 consecutive days prior to blood stem cell transplantation. Each BU dose was administered in 1 gelatine capsule to be swallowed and containing the individually calculated dose of pure BU active substance. Blood samples were obtained from 6 adult patients 0, 30, 60, 90, 120, 180, 240, 300, and 360 min after the 1st, 5th, and 13th BU dose, frozen and analyzed subsequently by using a HPLC assay with UV detection. In addition, in two patients concomitant TDM was executed. BU exposure was monitored concurrently and BU doses were targeted to achieve a steady-state plasma concentration (C(SS)) of 600-900 ng/mL or 900-1100 ng/mL depending on the underlying disease. In these patients blood samples were obtained 0, 60, 120, 180, 240, and 360 min after the 1st, 5th, 9th, and 13th BU dose and analyzed concurrently. RESULTS: For the six patients monitored retrospectively, the time to reach peak plasma BU concentration (C(max)) ranged from 1 to 5 h (mean 2.4 h). BU C(max) - values varied from 728 to 1807 ng/ mL (mean 1174 ng/mL), and BU clearance (CL/F) from 2.32 to 3.75 mL/min/kg (mean 2.97 mL/min/ kg). The mean BU steady state (C(SS)) concentration calculated was 973 ng/mL (range 754-1226 ng/mL) with a mean AUC of 5818 ng.h/mL (range 4521- 7171 ng.h/mL). One of the two patients receiving targeted BU doses required an upward dose adjustment. None of the eight patients suffered from vomiting during BU therapy. CONCLUSIONS: BU active substance encapsulated without further excipients in gelatine capsules is highly suitable for oral BU therapy. However, therapeutic drug monitoring and BU dose adjustment is still advisable to achieve optimal systemic BU exposure in each individual patient.

Mitomycin C aqueous humor concentration after photorefractive keratectomy: an experimental study.

PURPOSE: To evaluate mitomycin C (MMC) aqueous humor concentration after photorefractive keratectomy (PRK). METHODS: In this experimental study, twenty-four eyes of 12 male pigmented rabbits were divided into 4 groups and studied at the Institute of Vision and Optics, Department of Medicine, University of Crete, Greece. Eyes in groups 1 and 2 underwent PRK to correct -5 diopters (D) in a 6-mm optical zone, while sponges soaked with 0.02% MMC were applied on the exposed corneal stroma for 60 and 120 seconds, respectively. Similarly, eyes in groups 3 and 4 underwent PRK to correct -10 D in a 6-mm optical zone, while sponges soaked with 0.02% MMC were applied on the exposed corneal stroma for 60 and 120 seconds, respectively. Aqueous humor was extracted from all rabbit eyes 10 minutes after MMC application and high-performance liquid chromatography was performed immediately to detect and quantify MMC levels. RESULTS: The mean aqueous humor concentration of MMC was 0.23+/-0.03 microg/mL, 0.39+/-0.05 microg/mL, 0.28+/-0.04 microg/mL, and 0.52+/-0.16 microg/mL in groups 1, 2, 3, and 4, respectively. The effect of application time and correction on aqueous humor MMC concentration was significant (p<0.0001 and p=0.019), while the exposure time had a greater impact on aqueous humor MMC concentration when compared with the attempted correction. CONCLUSIONS: Both exposure time of MMC on the corneal stroma and the attempted correction was correlated with MMC aqueous humor concentrations.

In vivo and in vitro characterization of ethyl methanesulfonate pharmacokinetics in animals and in human.

Pharmacokinetics of ethyl methanesulfonate (EMS) were characterized in mice, rats and in cynomolgus monkeys with unlabelled and (14)C-radiolabelled EMS by either quantification of unchanged EMS or the N-ethyl-valine haemoglobin (Hb) adduct. EMS was well absorbed and exhibited close to 100% oral bioavailability. EMS showed some species differences in systemic clearance (intermediate in mice, low in rats and monkeys) representing only 1-4% of the cardiac output. The volume of distribution (0.5-0.8L/kg) was constant across species and corresponded to extracellular water. As a result of the species differences in clearance, the half-life ranged from 10 min in mouse (at low dose) to 5h in monkey. The systemic exposure of free EMS and the levels of its Hb adduct increased nearly dose proportionately from 1 to 5 and 0.5 to 80 mg/kg, respectively. The persistence of the N-ethyl-valine Hb adduct was much longer than EMS itself, consistent with the long life span of haemoglobin. No species differences were evident for the binding of EMS to Hb in whole blood ex vivo as determined by the second order rate constants. Following administration of Viracept tablets of the contaminated production batches (to monkeys leading to EMS doses of 0.08-27 microg/kg, concentrations of ethyl-valine Hb adducts) were near or below the detection limit of the assay (0.043 nmol/g Hb).