Imatinib pharmacokinetics in patients with gastrointestinal stromal tumour: a retrospective population pharmacokinetic study over time. EORTC Soft Tissue and Bone Sarcoma Group

Prediction of free imatinib concentrations based on total plasma concentrations in patients with gastrointestinal stromal tumours

AIM

Total imatinib concentrations are currently measured for the therapeutic drug monitoring of imatinib, whereas only free drug equilibrates with cells for pharmacological action. Due to technical and cost limitations, routine measurement of free concentrations is generally not performed. In this study, free and total imatinib concentrations were measured to establish a model allowing the confident prediction of imatinib free concentrations based on total concentrations and plasma proteins measurements.

METHODS

One hundred and fifty total and free plasma concentrations of imatinib were measured in 49 patients with gastrointestinal stromal tumours. A population pharmacokinetic model was built up to characterize mean total and free concentrations with inter-patient and intrapatient variability, while taking into account α1 -acid glycoprotein (AGP) and human serum albumin (HSA) concentrations, in addition to other demographic and environmental covariates.

RESULTS

A one compartment model with first order absorption was used to characterize total and free imatinib concentrations. Only AGP influenced imatinib total clearance. Imatinib free concentrations were best predicted using a non-linear binding model to AGP, with a dissociation constant Kd of 319 ng ml(-1) , assuming a 1:1 molar binding ratio. The addition of HSA in the equation did not improve the prediction of imatinib unbound concentrations.

CONCLUSION

Although free concentration monitoring is probably more appropriate than total concentrations, it requires an additional ultrafiltration step and sensitive analytical technology, not always available in clinical laboratories. The model proposed might represent a convenient approach to estimate imatinib free concentrations. However, therapeutic ranges for free imatinib concentrations remain to be established before it enters into routine practice.

Update on imatinib for gastrointestinal stromal tumors: duration of treatment

Gastrointestinal stromal tumors (GISTs) are the most common sarcoma of the gastrointestinal tract, with transformation typically driven by activating mutations of c-KIT and less commonly platelet-derived growth factor receptor alpha (PDGFRA). Successful targeting of c-KIT and PDGFRA with imatinib, a tyrosine kinase inhibitor (TKI), has had a major impact in advanced GIST and as an adjuvant and neoadjuvant treatment. If treatment with imatinib fails, further lines of TKI therapy have a role, but disease response is usually only measured in months, so strategies to maximize the benefit from imatinib are paramount. Here, we provide an overview of the structure and signaling of c-KIT coupled with a review of the clinical trials of imatinib in GIST. In doing so, we make recommendations about the duration of imatinib therapy and suggest how best to utilize imatinib in order to improve patient outcomes in the future.

Phase I trial, pharmacokinetics, and pharmacodynamics of vandetanib and dasatinib in children with newly diagnosed diffuse intrinsic pontine glioma

PURPOSE

Testing of promising drug combinations is crucial in the treatment of diffuse intrinsic pontine glioma (DIPG). As the VEGF and platelet-derived growth factor (PDGF) pathways are critical in gliomas, we evaluated the safety, maximum tolerated dose (MTD), pharmacokinetics, and pharmacodynamics of vandetanib, a VEGFR-2 inhibitor, combined with dasatinib, a potent PDGFR inhibitor, during and after radiotherapy in children with newly diagnosed DIPG.

EXPERIMENTAL DESIGN

Dasatinib was started concurrently with radiotherapy. Vandetanib was started 8 days later. We tested increasing doses of vandetanib (65 and 85 mg/m(2) once daily) and dasatinib (65 and 85 mg/m(2) twice daily). Dose-limiting toxicities were evaluated during the first 6 weeks of therapy. Plasma pharmacokinetics was obtained on days 8 and 42 ± 3 in all patients and concomitantly with cerebrospinal fluid (CSF) when possible. Inhibition of targets of dasatinib in peripheral blood mononuclear cells (PBMC) was evaluated.

RESULTS

Twenty-five patients were treated. Treatment was well tolerated. The median duration of treatment was 184 days. Diarrhea was the most significant toxicity. Three patients experienced substantial myelosuppression. The steady-state plasma pharmacokinetics of vandetanib was comparable with previous studies. Although the plasma exposure to dasatinib decreased from days 8 to 42, it remained similar to adult studies. CSF to plasma exposure of vandetanib and dasatinib were approximately 2% in 2 patients. Phosphorylated 70S6K decreased during therapy in PBMCs.

CONCLUSIONS

The MTD of vandetanib and dasatinib in combination was 65 mg/m(2) for each drug. Other studies are underway to test dasatinib and other PDGFR inhibitors alone or in combination for this deadly cancer.

Impact of genetic polymorphisms on chemotherapy toxicity in childhood acute lymphoblastic leukemia

The efficacy of chemotherapy in pediatric acute lymphoblastic leukemia (ALL) patients has significantly increased in the last 20 years; as a result, the focus of research is slowly shifting from trying to increase survival rates to reduce chemotherapy-related toxicity. At the present time, the cornerstone of therapy for ALL is still formed by a reduced number of drugs with a highly toxic profile. In recent years, a number of genetic polymorphisms have been identified that can play a significant role in modifying the pharmacokinetics and pharmacodynamics of these drugs. The best example is that of the TPMT gene, whose genotyping is being incorporated to clinical practice in order to individualize doses of mercaptopurine. However, there are additional genes that are relevant for the metabolism, activity, and/or transport of other chemotherapy drugs that are widely use in ALL, such as methotrexate, cyclophosphamide, vincristine, L-asparaginase, etoposide, cytarabine, or cytotoxic antibiotics. These genes can also be affected by genetic alterations that could therefore have clinical consequences. In this review we will discuss recent data on this field, with special focus on those polymorphisms that could be used in clinical practice to tailor chemotherapy for ALL in order to reduce the occurrence of serious adverse effects.

Evidence for Therapeutic Drug Monitoring of Targeted Anticancer Therapies

Therapeutic drug monitoring (TDM) provides valuable guidance for dose adjustment of antibiotics, immunosuppressives, antiepileptics, and other drugs, but its use for traditional anticancer therapies has been limited. Perhaps the most important obstacle is the impractical requirement of multiple blood samples to adequately define systemic exposure of drugs that have a short elimination half-life and are given by intermittent intravenous injections. However, the newer targeted anticancer therapies have different pharmacokinetic (PK) and dosing characteristics compared with traditional cytotoxic drugs, making it possible to estimate the steady-state drug exposure with a single trough-level measurement. Recent evidence indicates that certain PK parameters, including trough levels, are correlated with clinical outcomes for many of these agents, including imatinib, sunitinib, rituximab, and cetuximab. Although the current evidence is insufficient to mandate TDM in routine practice, a concerted investigation should be encouraged to determine whether the steady-state trough measurements of targeted agents will have a practical place in the clinical care of patients with cancer.

A long-term prospective population pharmacokinetic study on imatinib plasma concentrations in GIST patients

PURPOSE

Imatinib minimal (trough) plasma concentrations after one month of treatment have shown a significant association with clinical benefit in patients with gastrointestinal stromal tumors (GIST). Considering that a retrospective pharmacokinetic analysis has also suggested that imatinib clearance increases over time in patients with soft tissue sarcoma and GIST, the primary aim of this study was to assess systemic exposure to imatinib at multiple time points in a long-term prospective population pharmacokinetic study. As imatinib is mainly metabolized in the liver, our secondary aim was to elucidate the potential effects of the volume of liver metastases on exposure to imatinib.

EXPERIMENTAL DESIGN

Full pharmacokinetic blood sampling was conducted in 50 patients with GIST on the first day of imatinib treatment, and after one, six, and 12 months. In addition, on day 14, and monthly during imatinib treatment, trough samples were taken. Pharmacokinetic analysis was conducted using a compartmental model. Volume of liver metastases was assessed by computed tomographic (CT) imaging.

RESULTS

After 90 days of treatment, a significant decrease in imatinib systemic exposure of 29.3% compared with baseline was observed (P < 0.01). For every 100 cm(3) increase of metastatic volume, a predicted decrease of 3.8% in imatinib clearance was observed.

CONCLUSIONS

This is the first prospective pharmacokinetic study in patients with GIST, showing a significant decrease of approximately 30% in imatinib exposure after long-term treatment. This means that future "trough level - clinical benefit" analyses should be time point specific. GIST liver involvement, however, has a marginal effect on imatinib clearance.

Variability of sorafenib toxicity and exposure over time: a pharmacokinetic/pharmacodynamic analysis

BACKGROUND

Sorafenib displays major interpatient pharmacokinetic variability. It is unknown whether the pharmacokinetics of sorafenib influence its toxicity.

METHODS

We analyzed the severity and kinetics of sorafenib-induced toxicities in unselected consecutive patients with cancer, as well as their relationship with biological, clinical, and pharmacokinetic parameters. Toxicity was recorded bimonthly. Sorafenib plasma concentrations were assessed by liquid chromatography.

RESULTS

For 83 patients (median age, 62 years; range, 21-84 years), median sorafenib 12-hour area under the curve (AUC(0-12)) was 52.8 mg · h/L (range: 11.8-199.6). A total of 51 patients (61%) experienced grade 3-4 toxicities, including hand-foot skin reactions (23%), asthenia (18%), and diarrhea (11%). Sorafenib AUC(0-12) preceding grade 3-4 toxicities was significantly higher than that observed in the remaining population (61.9 mg · h/L vs. 53 mg · h/L). In 25 patients treated with fixed doses of sorafenib for the first 4 months, median dose-normalized AUC(0-12) on day 120 was significantly lower than on day 15 (63 vs. 102 mg · h/L). The incidence of hypertension and hand-foot skin reactions significantly decreased over time.

CONCLUSION

Sorafenib AUC(0-12) decreases over time, similarly to the incidence of hypertension and hand-foot skin reactions. Monitoring of sorafenib plasma concentrations may help to prevent acute severe toxicities and detect patients with suboptimal exposure at disease progression.

Contribution of tumoral and host solute carriers to clinical drug response

Members of the solute carrier family of transporters are responsible for the cellular uptake of a broad range of endogenous compounds and xenobiotics in multiple tissues. Several of these solute carriers are known to be expressed in cancer cells or cancer cell lines, and decreased cellular uptake of drugs potentially contributes to the development of resistance. As result, the expression levels of these proteins in humans have important consequences for an individual's susceptibility to certain drug-induced side effects, interactions, and treatment efficacy. In this review article, we provide an update of this rapidly emerging field, with specific emphasis on the direct contribution of solute carriers to anticancer drug uptake in tumors, the role of these carriers in regulation of anticancer drug disposition, and recent advances in attempts to evaluate these proteins as therapeutic targets.

Imatinib rechallenge in patients with advanced gastrointestinal stromal tumors

BACKGROUND

Imatinib is the standard of care for patients with advanced gastrointestinal stromal tumors (GIST).

DESIGN

This article reviews recent data on the impact of imatinib treatment interruption and subsequent rechallenge in patients with advanced GIST.

RESULTS

The randomized BFR14 trial showed that (i) interruption of imatinib after 1, 3, or 5 years of treatment in patients with nonprogressive GIST was associated with a high risk of progression even in patients with a complete response; (ii) rechallenge with imatinib restored tumor control in most patients, but the tumor response seldom reached that before treatment interruption; (iii) patients receiving continuous imatinib had a high rate of prolonged tumor control, which increased with longer imatinib treatment. The findings in the metastatic setting have important implications regarding the duration of adjuvant imatinib in GIST.

CONCLUSIONS

Discontinuation of imatinib in responding patients with advanced GIST is associated with a high risk of progression and is therefore not recommended. Although rechallenge is a strategy for treating patients who relapse after stopping imatinib, suboptimal tumor response indicates that continuous kinase suppression is necessary to achieve the best clinical outcome. Three-year adjuvant imatinib is recommended for patients with resected 'high-risk' GIST; however, a longer duration may provide additional benefits.

Adjuvant imatinib therapy for gastrointestinal stromal tumors

Surgery is the standard of care for primary resectable gastrointestinal stromal tumors (GISTs), but half of surgically treated patients relapse. Imatinib (IM) has been shown to prolong recurrence-free survival after complete surgery and is now approved as adjuvant therapy (400  mg/day) for high-risk GIST patients. IM is well tolerated, with mild to moderate side effects observed. Whether adjuvant IM prolongs overall survival is under evaluation in two ongoing clinical trials.

Imatinib mesylate: past successes and future challenges in the treatment of gastrointestinal stromal tumors

Just over a decade ago, gastrointestinal tumours were a poorly understood mesenchymal neoplasm unsuccessfully treated with chemotherapy. Cytotoxic therapy for advanced disease yielded response rates of 10% and median survival of just 18 months. However, the discovery of KIT and platelet derived growth factor receptor alpha (PDGFRA) mutations as oncogenic drivers of most gastrointestinal tumours, paved the way for targeted therapy. Imatinib mesylate, a tyrosine kinase inhibitor, produces a clinical benefit rate (complete response, partial response, and stable disease) of more than 80% in metastatic setting and a median survival of 57 months. Imatinib is now also approved in adult patients following resection of KIT-positive GIST. Major insights into the mechanism of action of imatinib, unique pharmacokinetics, drug resistance, and management of low grade but chronic adverse effects continue to be made.

Sorafenib exposure decreases over time in patients with hepatocellular carcinoma

BACKGROUND

Intra-patient variability in sorafenib pharmacokinetics has been poorly investigated to date. We hypothesized that sorafenib clearance could decrease over time, as seen with imatinib.

PATIENTS AND METHODS

Sorafenib plasma concentrations were determined by liquid chromatography, every 2 weeks, in consecutive hepatocellular carcinoma patients treated with sorafenib. Sorafenib dose-normalized area under the concentration-time curve (AUC) was determined from a population pharmacokinetics model, and its kinetics was analyzed in order to identify possible alterations of exposure over time.

RESULTS

Fifteen hepatocellular carcinoma patients with Child-Pugh A cirrhosis, in whom sorafenib dosing remained unchanged from initiation of treatment to disease progression, were eligible for this analysis. Sorafenib AUC significantly decreased over time: the median AUC during the third month of treatment was lower than that observed after one month of treatment (43.0 vs. 60.3 mg/L.h, p = 0.008). Most importantly, median sorafenib AUC at the time of progression was almost two-fold lower than that observed after one month of therapy (33.2 vs. 60.3 mg/L.h, p = 0.007). These findings suggest an induction of expression of efflux transporters in the gut wall, or an induction of sorafenib metabolism.

CONCLUSIONS

In patients with progressive disease in whom exposure markedly decreased from baseline, sorafenib dose escalation could be considered, aiming to restore an adequate drug exposure and possibly anti-tumor activity.

Genetic determinants of anticancer drug activity: towards a global approach to personalized cancer medicine

While current trials of anticancer agents serve to provide a population-based validation of therapeutic activity, clinical success is typically restricted to tumors of select molecular subtype. Recent insights have yielded a growing catalogue of germline and tumor-based aberrations that can predetermine whether a patient will achieve clinical benefit from a drug or not. Thus, in order to realize the true potential of anticancer agents, we need to define the molecular contexts under which they will prove both efficacious and safe. In this article, we provide an overview of such molecular determinants and introduce the concept of 'cancer patient profiling' - the process and science of defining the optimal therapy for a given patient through the generation and analysis of system-wide molecular information.

Treatment of gastrointestinal stromal tumor: focus on imatinib mesylate

Gastrointestinal stromal tumor (GIST) is a rare primary neoplasm of the gastrointestinal tract, mesentery, or omentum. In the past, surgery has been the only effective treatment. The diagnosis and treatment of GIST has been revolutionized over the past decade, since expression of the receptor tyrosine kinase KIT was shown to occur on these tumors. Mutations in this proto-oncogene commonly cause constitutive activation of the KIT tyrosine kinase receptor, an important factor in the pathogenesis of the disease. The development of specific tyrosine kinase inhibitors, such as imatinib mesylate, has led to a breakthrough in the treatment of advanced GIST. Treatment with this drug has led to significant improvements in survival, with overall response rates in excess of 80%. Side effects are common, but usually manageable. The success of this drug has led to further trials investigating its use in the pre- and postoperative situation. This review summarizes the current knowledge of GIST and imatinib treatment and possible future developments.

Pharmacokinetic impact of SLCO1A2 polymorphisms on imatinib disposition in patients with chronic myeloid leukemia

The purpose of this study was to explore the role of the organic anion-transporting polypeptide (OATP) 1A2, which is encoded by SLCO1A2, in the cellular uptake of the Bcr-Abl tyrosine kinase inhibitor imatinib, and the relationship between SLCO1A2 polymorphisms and the pharmacokinetics of imatinib in patients with chronic myeloid leukemia (CML). Imatinib uptake was significantly enhanced in OATP1A2-transfected human embryonic kidney (HEK) 293 cells (P = 0.002). Naringin, an OATP1A2 inhibitor, decreased the transport of imatinib in OATP1A2-transfected HEK293 cells, the human intestinal cell line Caco-2, and K562 CML cells. Linkage disequilibrium was found between the SLCO1A2 -1105G>A and -1032G>A genotypes in 34 CML patients and 100 healthy subjects. Imatinib clearance in CML patients was influenced by the SLCO1A2 -1105G>A/-1032G>A genotype (P = 0.075) and the SLCO1A2 -361GG genotype (P = 0.005). These findings suggest that imatinib is transported into cells by OATP1A2, and that SLCO1A2 polymorphisms significantly affect imatinib pharmacokinetics.

Environmental and genetic factors affecting transport of imatinib by OATP1A2

The bioavailability of orally administered imatinib is >90%, although the drug is monocationic under the acidic conditions in the duodenum. In vitro, we found that imatinib is transported by the intestinal uptake carrier organic anion transporting polypeptide (OATP1A2) and that this process is sensitive to pH, rosuvastatin, and genetic variants. However, in a study in patients with cancer, imatinib absorption was not associated with OATP1A2 variants and was unaffected by rosuvastatin. These findings highlight the importance of verifying in a clinical setting the drug-transporter interactions observed in in vitro tests.

Imatinib: as adjuvant therapy for gastrointestinal stromal tumour

Imatinib is a protein-tyrosine kinase inhibitor with antitumour effects in patients with gastrointestinal stromal tumour (GIST) that is indicated for the treatment of unresectable and/or metastatic GIST and as adjuvant therapy in patients with KIT-positive GIST. Imatinib binds to and inhibits KIT and platelet-derived growth factor receptor (PDGFR)-α tyrosine kinases, interfering with their downstream tumourogenic processes. Cell lines with KIT mutations that are common in patients with GIST were sensitive to imatinib at low in vitro concentrations. Patients with exon 11 KIT mutations were significantly more likely to have partial tumour responses and longer overall survival (OS) and were significantly less likely to have progression of disease than patients with exon 9 KIT mutations or no detectable KIT or PDGFR mutations. In a large (n > 700) randomized, double-blind, placebo-controlled, multinational trial (ACOSOG [American College of Surgeons Oncology Group] Z9001), patients who received 1 year of adjuvant treatment with oral imatinib 400 mg/day after surgical resection of GIST had significantly longer recurrence-free survival than placebo recipients, with an overall hazard ratio of 0.35 (95% CI 0.22, 0.53) [primary endpoint]. At the time of reporting, there was no significant between-group difference in OS. In patients with GIST who received adjuvant imatinib in this trial, adverse events were mostly of mild or moderate severity; the imatinib treatment group had an almost 2-fold higher rate of US National Cancer Institute Common Toxicity Criteria grade 3 or 4 adverse events than the placebo group.

The role of imatinib plasma level testing in gastrointestinal stromal tumor

The management of patients with gastrointestinal stromal tumor (GIST) has markedly advanced over the past 10 years. Imatinib has exceptional activity in controlling gastrointestinal stromal tumor (GIST) due to inhibition of the constitutively active conformation of KIT and PDGFRA which is found in the majority of patients with GIST. Although some patients may experience prolonged disease control while on imatinib, most patients will develop imatinib resistance within 2-3 years on therapy. A recent retrospective analysis demonstrated a relationship between imatinib plasma levels and progression-free survival in patients with advanced GIST. Plasma imatinib levels in this study were unrelated to the daily administered dose of imatinib. A prospective trial is underway in order to evaluate whether modification of imatinib dose to achieve a target imatinib plasma level will impact patient outcome when compared to standard imatinib dosing in GIST ( http://www.clinicaltrials.gov , NCT01031628).  This review will explore the current available data on the relationship between imatinib plasma levels, response to treatment, and other prognositic factors as well as discuss the implications of this data for possible future therapeutic approaches.

Correlations between imatinib pharmacokinetics, pharmacodynamics, adherence, and clinical response in advanced metastatic gastrointestinal stromal tumor (GIST): an emerging role for drug blood level testing?

Imatinib is the standard of care for patients with advanced metastatic gastrointestinal stromal tumors (GIST), and is also approved for adjuvant treatment in patients at substantial risk of relapse. Studies have shown that maximizing benefit from imatinib depends on long-term administration at recommended doses. Pharmacokinetic (PK) and pharmacodynamic factors, adherence, and drug-drug interactions can affect exposure to imatinib and impact clinical outcomes. This article reviews the relevance of these factors to imatinib's clinical activity and response in the context of what has been demonstrated in chronic myelogenous leukemia (CML), and in light of new data correlating imatinib exposure to response in patients with GIST. Because of the wide inter-patient variability in drug exposure with imatinib in both CML and GIST, blood level testing (BLT) may play a role in investigating instances of suboptimal response, unusually severe toxicities, drug-drug interactions, and suspected non-adherence. Published clinical data in CML and in GIST were considered, including data from a PK substudy of the B2222 trial correlating imatinib blood levels with clinical responses in patients with GIST. Imatinib trough plasma levels < 1100 ng/mL were associated with lower rates of objective response and faster development of progressive disease in patients with GIST. These findings have been supported by other analyses correlating free imatinib (unbound) levels with response. These results suggest a future application for imatinib BLT in predicting and optimizing therapeutic response. Nevertheless, early estimates of threshold imatinib blood levels must be confirmed prospectively in future studies and elaborated for different patient subgroups.

Evaluation of self-reported progression and correlation of imatinib dose to survival in patients with metastatic gastrointestinal stromal tumors: an open cohort study

Objectives

Self-reported progression was evaluated as a predictor of survival in patients with metastatic gastrointestinal stromal tumor (GIST).

Methods

This is a follow-up of an open cohort study of Life Raft Group (LRG) members with a diagnosis of KIT-positive metastatic GIST receiving imatinib from May 2000–December 2007 reporting their subjective response to therapy by completion of an internet-based questionnaire. Subjects received ≥ 1 year of imatinib and reported an initial positive response. Members reporting stable disease or progression were excluded. Self-reported progression-free survival (srPFS) was compared with overall survival (OS) and analyzed by starting and last reported dose.

Results

One hundred sixty-nine subjects reported a mean starting dose of 527.8±177.9 mg/d at a mean age of 53.8±11.6 years at initial diagnosis. Of those reporting progression, 66% died versus 11% of those not reporting progression (P < 0.0001). When analyzed by last reported dose, a median srPFS benefit of 27.3 months was observed for the >400 mg/d group (P = 0.0017). Sixty-two percent of subjects who initiated therapy at >400 mg/d reported a dose reduction. When analyzed by last reported dose, a significant benefit in OS (P = 0.0229) and srPFS (P = 0.0069) was observed for subjects taking 600 over 400 mg/d.

Conclusions

srPFS strongly correlated with OS. Significant advantages were observed when last reported dose was considered, as was higher daily dose. These observations suggest that careful escalation to intermediate daily doses should be investigated further for its potential to reduce the incidence and severity of adverse events, but also as a strategy against developing secondary resistance to imatinib.

The role of high-dose imatinib in the management of patients with gastrointestinal stromal tumor

After an era of few treatment options for patients with locally advanced or metastatic gastrointestinal stromal tumor (GIST), imatinib has emerged as the standard of care and first-line treatment for these patients. Although imatinib was initially approved at the doses of 400 and 600 mg daily, results from clinical studies established 400 mg daily as the standard initial dose for the majority of advanced GIST patients. Nevertheless, the use of high-dose imatinib (800 mg daily) has been shown to benefit patients with advanced or metastatic GIST that progresses on the standard-dose, and has been recommended in this setting by the major management guidelines in Europe and the United States. Results from the Meta-GIST meta-analysis showed that patients whose GIST harbors a KIT exon 9 mutation garner a longer progression-free survival time when treated initially with high-dose imatinib (800 mg daily) compared with those patients with KIT exon 11 or no mutations. Thus, the use of high-dose imatinib is recommended by the clinical practice guidelines in these 2 specific clinical situations. In addition, clinicians should weigh the clinical benefit of administering high-dose imatinib against the associated toxicities, as well as the proper management of dose-related side effects.

Targeted therapy for gastrointestinal stromal tumors: current status and future perspectives

Gastrointestinal stromal tumors (GISTs) present 80% of gastrointestinal tract mesenchymal tumors, with systemic chemotherapy and radiotherapy being unable to improve survival of patients with advanced disease. The identification of activating mutations in either KIT cell surface growth factor receptor or platelet-derived growth factor receptor alpha, which lead to ligand-independent signal transduction, paved the way for the development of novel agents that selectively inhibit key molecular events in disease pathogenesis. The development of imatinib mesylate in the treatment of metastatic GIST represents a therapeutic breakthrough in molecularly targeted strategies, which crucially improved patients' prognosis while its usefulness in adjuvant and neoadjuvant setting is under study. Sunitinib malate is available in the second-line setting, with ongoing studies evaluating its role in an earlier disease stage, while other targets are under intense investigation in order to enrich the therapeutical armamentarium for this disease. GIST phenotype seems to be an essential indicator of treatment response; thus, obtaining genotype information of each patient may be critical in order to tailor individualized treatment strategies and achieve maximal therapeutic results.

Optimal use of targeted agents for advanced gastrointestinal stromal tumours

Imatinib is the recommended 1st-line treatment for a KIT-positive unresectable and/or metastatic gastrointestinal stromal tumour (GIST). However, some patients experience intolerance to imatinib and most patients will eventually experience disease progression while on imatinib treatment. Sunitinib is approved for treatment of a GIST after disease progression on, or intolerance to, imatinib therapy. Progression may occur early or later on, in treatment and is determined by factors including initial GIST genotype and mutational status. GISTs with KIT exon 11 mutations appear to be sensitive to standard dose imatinib, and patients with GISTs exhibiting KIT exon 9 mutations whose disease has progressed on imatinib 400 mg/day have been shown to respond to imatinib 800 mg/day, albeit with a higher incidence of adverse events. Sunitinib has shown clinical benefit in all major GIST mutational subtypes, particularly in patients with wild-type or KIT exon 9 genotype and against GISTs with secondary KIT exon 13 or 14 mutations. The choice between higher-dose imatinib and sunitinib after progression on standard dose imatinib is unclear, and apart from the GIST primary resistance genotype and mutational status, individual patient factors such as tumour characteristics, drug pharmacokinetics, and other clinical factors may affect response to treatment. Individualisation of therapy may help to maximise clinical benefit of therapy in these patients.

Targeted therapies of gastrointestinal stromal tumors (GIST)--the next frontiers

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal (GI) tract and are caused by activating KIT or PDGFRA mutations. GISTs can be successfully treated with the small molecule kinase inhibitor imatinib mesylate (Gleevec, Novartis) with response rates of up to 85%. However, complete responses are rare, and most patients will develop imatinib resistance over time. Recent results have shown that although imatinib effectively stimulates apoptotic cell death in sensitive GIST cells, a considerable proportion of cells does not undergo apoptosis, but instead enters a state of quiescence. Quiescence is characterized by a reversible withdrawal from the cell division cycle, during which the cells remain alive and metabolically active. It is conceivable that quiescence not only plays a pivotal role in the emergence of residual disease but also in creating a pool of tumor cells that survive continuous small molecule therapy and may hence represent the "seeds" for the outgrowth of resistant clones. This review will summarize the current knowledge about GIST biology and treatment response to imatinib including the induction of cellular quiescence in GIST. In addition, we will highlight future strategies to design more effective treatment options to overcome these problems with an aim towards cure of this hitherto untreatable tumor entity.

Clinical responses observed with imatinib or sorafenib in melanoma patients expressing mutations in KIT

Background:

Mutations in KIT are more frequent in specific melanoma subtypes, and response to KIT inhibition is likely to depend on the identified mutation.

Methods:

A total of 32 patients with metastatic acral or mucosal melanoma were screened for mutations in KIT exons 11, 13 and 17.

Results:

KIT mutations were found in 38% of mucosal and in 6% of acral melanomas. Three patients were treated with imatinib and one with sorafenib. All four patients responded to treatment, but three have since progressed within the brain.

Conclusion:

The observed clinical responses support further investigation of KIT inhibitors in metastatic melanoma, selected according to KIT mutation status.

Cross-Sectional Study of Imatinib Plasma Trough Levels in Patients With Advanced Gastrointestinal Stromal Tumors: Impact of Gastrointestinal Resection on Exposure to Imatinib

Purpose

Imatinib plasma trough levels (Cmin) have been reported to correlate with treatment outcomes in patients with gastrointestinal stromal tumors (GISTs). We therefore have evaluated the correlation between imatinib Cmin and the clinical characteristics of patients with GIST.

Patients and Methods

Steady-state imatinib Cmin in 107 patients with GIST who were taking imatinib 300 to 800 mg/d was measured.

Results

In patients treated with imatinib 400 (n = 92), 300 (n = 7), 600 (n = 2), or 800 (n = 11) mg/d, imatinib Cmin was 1,305 ± 633 ng/mL, 1,452 ± 830 ng/mL, 1,698 ± 725 ng/mL, and 3,330 ± 1,592 ng/mL, respectively. Of the 92 patients treated with 400 mg/d imatinib, 59 patients (63%) were men; the median age was 55 years (range, 28 to 76 years), and the median duration of imatinib use before sampling was 8.8 months (range, 0.5 to 67.6 months). The mean inter- and intrapatient variability rates were 44.7% and 26.5%, respectively. In univariate analyses, high Cmin was correlated with advanced age (P = .02), low creatinine clearance (P = .001), low hemoglobin (P = .03), and low albumin (P < .001) concentrations. Imatinib Cmin was also significantly lower in patients with (n = 18; 942 ± 330 ng/mL) than without (n = 74; 1,393 ± 659 ng/mL) major (ie, total or subtotal) gastrectomy (P = .002). In multivariate analysis, albumin (P = .001) concentrations, creatinine clearance (P = .002), and major gastrectomy (P = .003) were significantly correlated with Cmin.

Conclusion

In patients with GIST, imatinib Cmin at steady state was significantly associated with albumin concentration, creatinine clearance, and previous major gastrectomy. Although its clinical impact is unclear at present time, monitoring of imatinib Cmin might be particularly important for optimal treatment with imatinib in patients who have undergone major gastrectomy.

Interleukin-2 treatment effect on imatinib pharmacokinetic, P-gp and BCRP expression in mice

The aim of this study was to investigate the effect that recombinant interleukin-2 (rIL-2) (0.16 MUI/injection) had on the pharmacokinetics of imatinib (IM) in plasma. In this study, IM was given orally to mice at a dose of 150 mg/kg once a day for 11 days (from day 1 to 11) either alone or in combination with intraperitoneal injections of rIL-2 twice a day from day 8 to 11. Pharmacokinetic parameters were determined using WinNonLin software. Areas under the curve were compared using Bailer's method. The repeated administration of the rIL-2+IM combination was shown to have two pharmacokinetic advantages compared with repeated IM doses alone. In addition to the pharmacodynamic interest of this treatment, we found that the combined treatment significantly increased the IM Cmax (P<0.05) and significantly increased the IM trough concentration (C(24 h)) (P<0.01), which was always above the minimum therapeutic IM concentration (1 mumol/l) in plasma. Those pharmacokinetic modifications may be explained, in part, by a decrease in the P-glycoprotein expression in the three intestinal segments of the mice (duodenum, P<0.01; jejunum, P<0.05; and ileum, P<0.05) and a decrease in BCRP expression in the duodenum segment (P<0.05) due to rIL-2. In another experiment, we found a significant induction of intestinal P-glycoprotein expression in mice that had been given IM orally (150 mg/kg) twice a day for 11 days. It would be interesting to further investigate the IM disposition associated with rIL-2 treatment for clinical applications.

A therapeutic drug monitoring algorithm for refining the imatinib trough level obtained at different sampling times

BACKGROUND

Correlation analyses have demonstrated that maintaining an adequate imatinib (IM) trough concentration would be important for clinical response in patients with chronic myeloid leukemia (CML) and Kit-positive gastrointestinal stromal tumors. The objectives of the current work were to use a pharmacokinetic model to refine the trough levels obtained at different sampling times and to propose a therapeutic drug monitoring algorithm and an acceptable sampling time window for imatinib trough sampling.

METHODS

The pharmacokinetics of IM in patients (pts) with CML were characterized based on historical data from a Phase III study. In the elimination phase the concentration of IM (C(t)) follows a mono-exponential decline, and the standardized trough concentration (C(min,std) = C(tau)) can be described by a simple algorithm C(min,std) = C(t)* exp(k(e) x Delta t), where Delta t = t - tau, and tau is 24 hours for qd or 12 hours for bid dosing and k(e) is the elimination rate constant. The percent deviation of C(t) from C(min,std) was simulated for different Delta t and k(e) values to define a sampling time window Delta t, within which the percent deviation is <20%.

RESULTS

Simulation analysis shows that C(t) is largely dependent on Delta t and k(e). The percent deviation of C(t) at 3 hours before or after tau from C(min,std) will be 7.1%, 13.1%, and 23.4% for pts with low, typical, and high k(e) values, 0.023/hour, 0.041/hour, and 0.070/hour, respectively. However, if a correction is made for C(t) by the algorithm using the typical k(e) value of 0.041 per hour, the percent deviation at 3 hours will be reduced to 5.3%, 0%, and 9.1% for pts with low, typical, and high k(e) values, respectively. Even if the sampling window is extended to +/-6 hours, the corresponding percent deviation will still be reasonable: 10.2%, 0%, and 19.0%, respectively.

CONCLUSION

By using the algorithm, the pharmacokinetic sampling window can be extended to a wider window to make the trough sampling easy to implement in the clinical setting, provided that the sampling time and dosing time are accurately recorded.

Evaluation of imatinib mesylate in the treatment of pulmonary arterial hypertension

Imatinib mesylate is a small molecule inhibitor that selectively inhibits the PDGF receptor kinase as well the cKIT and Abl kinases, among other targets. Various studies have implicated the PDGF pathway in the pathogenesis of pulmonary arterial hypertension (PAH). Inhibition with imatinib mesylate has shown efficacy in human case reports and experimental models of PAH. Results from a Phase II trial of imatinib mesylate in PAH did not meet the primary end point but showed improvement in several secondary end points and in a subgroup analysis. As suggested by this study as well as a few case reports, imatinib may be effective in a subset of patients with more severe disease. However, this remains to be further validated through a Phase III study, which is already underway. In conclusion, it appears that imatinib mesylate may hold promise as an adjunct drug in PAH therapy, especially since it is directed at a pathway not previously targeted.

Phase II study of neoadjuvant imatinib in glioblastoma: evaluation of clinical and molecular effects of the treatment

PURPOSE

Phase I-II studies indicate that imatinib is active in glioblastoma multiforme. To better understand the molecular and clinical effects of imatinib in glioblastoma multiforme, we conducted a neoadjuvant study of imatinib with pretreatment and posttreatment biopsies.

EXPERIMENTAL DESIGN

Patients underwent a computerized tomography-guided biopsy of their brain tumors. If diagnosed with glioblastoma multiforme, they were immediately treated with 7 days of imatinib 400 mg orally twice daily followed by either definitive surgery or re-biopsy. Pretreatment and posttreatment tissue specimens were tested by immunohistochemistry for Ki67 and microvessel destiny, and posttreatment specimens were analyzed for the presence of intact imatinib in tissue. Furthermore, pretreatment and posttreatment pairs were analyzed by Western blotting for activation of platelet-derived growth factor receptor, epidermal growth factor receptor (EGFR), phosphoinositide 3-kinase/AKT, and mitogen-activated protein kinase signaling pathways. Pharmacokinetic studies were also done.

RESULTS

Twenty patients were enrolled. Median survival was 6.2 months. Intact imatinib was detected in the posttreatment tissue specimens using mass spectrometry. There was no evidence of a drug effect on proliferation, as evidenced by a change in Ki67 expression. Biochemical evidence of response, as shown by decreased activation of AKT and mitogen-activated protein kinase or increased p27 level, was detected in 4 of 11 patients with evaluable, matched pre- and post-imatinib biopsies. Two patients showed high-level EGFR activation and homozygous EGFR mutations, whereas one patient had high-level platelet-derived growth factor receptor-B activation.

CONCLUSIONS

Intact imatinib was detected in glioblastoma multiforme tissue. However, the histologic and immunoblotting evaluations suggest that glioblastoma multiforme proliferation and survival mechanisms are not substantially reduced by imatinib therapy in most patients.