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Background Regorafenib at the standard intermittent dosing schedule proved effective in the GRID trial for refractory gastrointestinal stromal tumors (GISTs). However, this dosing schedule requires frequent dose reduction, and the progression of GISTs or tumor-related symptoms during the off-treatment period has also been noted in some patients. Therefore, we conducted this phase II trial to evaluate the efficacy and safety of regorafenib at a lower dose on a continuous dosing schedule. Methods Patients with measurable, metastatic, or recurrent GISTs who failed to respond to both imatinib and sunitinib were eligible for this study. Regorafenib 100 mg p.o. daily was administered continuously. The primary endpoint was disease control rate (DCR: complete response plus partial response [PR] plus stable disease [SD]) lasting for at least 12 weeks using RECIST version 1.1. Results The best response was PR in 2 (8%), SD in 16 (64%), and progressive disease in 6 (24%) patients. DCR lasting for at least 12 weeks was 64% (16 of 25). The median progression-free survival was 7.3 months (95% confidence interval, 5.9–8.6), and the 1-year survival rate was 64.5%. Ten patients (40%) experienced grade 3–4 toxicities, including hand-foot skin reaction (n = 4, 16%) and elevation of alanine aminotransferase (n = 2, 8%). Only six patients (24%) needed dose modification with a relative dose intensity of 95.0% for eight cycles in all patients. Conclusion Regorafenib at a lower dose on a continuous schedule might be an alternative treatment in patients with GISTs after failure of imatinib and sunitinib. Clinical trial identification number. NCT02889328 Implications for Practice Regorafenib at the standard intermittent dosing schedule proved effective in the GRID trial for refractory gastrointestinal stromal tumors (GISTs). However, this dosing schedule requires frequent dose reduction, and the progression of GISTs or tumor-related symptoms during the off-treatment period has been noted in some patients. This study was to evaluate the efficacy and safety of regorafenib at a lower dose on a continuous dosing schedule. With good efficacy and acceptable safety profiles, regorafenib at a lower, continuously administered dose might be an alternative treatment in patients with GISTs after imatinib and sunitinib. Rechallenge of regorafenib may slow the disease progression.

11539 Background: Although current guidelines recommend indefinite imatinib treatment in advanced gastrointestinal stromal tumor (GIST) patients, prolonged imatinib treatment is often challenging. Imatinib-refractory progression-free survival (PFS) and overall survival were reportedly not different between those who interrupted imatinib and those did not. Methods: Clinical outcomes of 77 consecutive patients with recurrent or metastatic GIST who interrupted imatinib treatment after maintaining around 5 years of imatinib treatment in the absence of gross tumor lesions were retrospectively analyzed. Association between clinical factors and progression-free survival (PFS) following imatinib interruption was analyzed. Results: Median time from the absence of gross tumor lesions to imatinib interruption was 61.5 months (range 41.4–122.7 months). Since imatinib interruption, median PFS was 19.6 months, and the PFS rate at 60 months was 26.3%. Among patients who had progressive disease following interruption, imatinib re-introduction led to 88.6% objective response rate and 100% disease control rate, with the median imatinib-refractory PFS since re-introduction of 112.4 months. Complete removal of initial gross tumor lesion(s) (hazard ratio [HR] 0.32, 95% confidence interval [CI] 0.12–0.81, P = 0.016) and complete removal of residual gross tumor lesion(s) (HR 0.45, 95% CI 0.21–0.94, P = 0.033) by local treatment (vs. no local treatment or residual lesions after local treatment) were independently associated with favorable PFS. Conclusions: Interruption of imatinib may be considered with close surveillance in selected patients who maintained years of imatinib treatment in the absence of gross tumor lesions, especially for those who had complete removal of gross tumor lesions by local treatment.

Background To achieve optimal clinical outcomes in patients with gastrointestinal stromal tumor (GIST), it is crucial to maintain sufficient dosing of imatinib. Skin rash is a common imatinib-associated adverse event and may affect compliance. This phase II study was conducted to evaluate whether imatinib-associated severe skin rash can be managed with systemic steroids without dose reduction or interruption of imatinib. This study is registered at ClinicalTrials.gov, number NCT03440515. Patients and Methods Between 2014 and 2016, 29 patients with imatinib-associated severe skin rash were enrolled. Skin rash of grade 2 with grade ≥2 pruritus or of grade 3 was considered severe. Oral prednisolone was administered 30 mg/day for 3 weeks, then tapered off over 12 weeks. The primary endpoint was treatment success rate (TSR). Treatment success was defined as maintaining imatinib for more than 15 weeks after completion of the steroid administration schedule without skin rash that led to additional steroid treatment or dose reduction or interruption of imatinib. Results Of the 29 patients enrolled, 22 patients with skin rash were treated successfully (TSR, 75.8%), 2 (6.9%) were evaluated as treatment failures, and 5 (17.2%) were not evaluable. The 2-year rash-free and imatinib reduction-free interval rate was 67.2% with median follow-up of 22.0 months (range, 0.4–30.3). Recurrence of severe skin rash occurred in seven patients (24.1%). Systemic steroids were well tolerated except in one patient who experienced pneumocystis pneumonia. Conclusion This study demonstrated that imatinib-associated severe skin rash can be effectively controlled by systemic steroid treatment without interruption or dose reduction of imatinib in patients with GIST. Implications for Practice Imatinib has been the standard treatment of gastrointestinal stromal tumor in both adjuvant and palliative settings. It is crucial to maintain sufficient dosing of imatinib to achieve optimal clinical outcomes. Imatinib commonly causes imatinib-associated skin rash, which may worsen drug compliance. This phase II study demonstrated that systemic steroids could help maintaining the efficacy of imatinib by preventing interruption or dose reduction of imatinib. The present study provides a new administration strategy of systemic steroids and its efficacy and safety data. Thus, this study can be a cornerstone to establish treatment guidelines for imatinib-associated skin rash.

Purpose This study evaluated the incidence of imatinib-associated skin rash, the interventional outcomes of severe rash, and impact of severe rash on the outcomes of imatinib treatment in gastrointestinal stromal tumor (GIST) patients. Materials and methods A total of 620 patients were administered adjuvant or palliative imatinib for GIST at Asan Medical Center between January 2000 and July 2012. This analysis focused on a group of 42 patients who developed a severe rash requiring major interventions, defined as dose interruption or reduction of imatinib or systemic steroid use. Results Of the 620 patients treated with imatinib, 148 patients (23.9%) developed an imatinib-associated skin rash; 42 patients (6.8%) developed a severe rash requiring major intervention. Of these, 28 patients (66.8%) successfully continued imatinib with interventions. Serial blood eosinophil levels during imatinib treatment were associated with skin rash and severity. A significant association was observed between successful intervention and blood eosinophil level at the time of intervention initiation. In metastatic settings, patients with severe rash requiring major interventions tended to show poorer progression-free survival than patients who did not require major intervention and patients with no rash, although this finding was not statistically significant (p=0.326). Conclusion By aggressive treatment of severe rash through modification of imatinib dose or use of systemic steroid, the majority of patients can continue on imatinib. In particular, imatinib dose intensity can be maintained with use of systemic steroid. Measuring the blood eosinophil levels may be helpful in guiding the management plan for skin rash regarding the intensity and duration of interventions.

A pharmacokinetic study in patients with gastrointestinal stromal tumors (GIST) suggested that imatinib plasma concentration may decrease following long-term exposure. We assessed changes in imatinib plasma trough levels (C(min)) during long-term treatment. Follow-up (FU) imatinib C(min) was measured in 65 patients who received the same dose of imatinib for at least 9 months after previous (initial) tests. After exclusion of 7 patients who had been treated with imatinib for over 2 years at the time of initial testing, 58 patients were included in this analysis. The median intervals from initiation of imatinib to initial testing and from initial to FU testing were 5.5 months (range, 0.5-24.0 months) and 13.0 months (range, 9.6-17.9 months), respectively. Mean inter- and intra-subject variability values were 47.7% and 20.9%, respectively, at initial measurements, and 45.2% and 19.4%, respectively, at FU. Mean FU imatinib C(min) (1,370 ± 661 ng/mL) was significantly higher than mean initial C(min) (1,171 ± 573 ng/mL; p = 0.003). Compared with initial C(min), FU C(min) was decreased in 22 patients and increased in 36, with median changes of 13% and 32%, respectively. Multivariate analysis showed a significant correlation between the ratio of FU to initial imatinib C(min) and that of albumin (r = -0.39, p = 0.003). During long-term treatment, imatinib C(min) did not decrease significantly but remained stable or increased in most patients. Changes in imatinib C(min) were associated with changes in albumin concentration. Monitoring of imatinib C(min) only for concerns about time-dependent increases in imatinib clearance is not necessary.

11537Background: Regorafenib in the standard intermittent dosing schedule (160 mg po per day for 3 weeks followed by 1week rest) was proven effective in the GRID trial in refractory gastrointestina...

Imatinib dose escalation has been suggested as an effective therapy for advanced gastrointestinal stromal tumors (GIST) after progression on the standard dose. We evaluated the efficacy, tolerability, and pharmacokinetics of imatinib dose escalation. Eighty-four patients with GIST who received imatinib 800 mg/day as second-line therapy were reviewed. In 66 patients, imatinib plasma trough level (Cmin) at 800 mg/day was measured. The relationships between imatinib exposure and therapeutic efficacy or toxicity were examined by grouping patients into quartiles according to Cmin and its percent change after dose escalation. Disease control was achieved in 56 % of patients. The median progression-free survival (PFS) was 5.1 months. There was a strong tendency for better PFS in patients with KIT exon 9 mutations compared to patients with other genotypes (median PFS 11 vs 4 months, p = 0.051). The common grade 3–4 toxicities were anemia (26 %), neutropenia (11 %), and hemorrhage (5 %). Mean ± standard deviation imatinib Cmin at 800 mg/day and percent Cmin change was 3,552 ± 1,540 ng/mL and 160 ± 101 %, respectively. Body surface area, hemoglobin, and absolute neutrophil count were independent covariates of Cmin at 800 mg/day. Neither Cmin nor its percent change associated with efficacy. The upper three quartiles of percent Cmin change associated with more frequent severe toxicities (56 %) than the lowest quartile (10 %; p = 0.01). Dose escalation to 800 mg/day was active and feasible in GIST after progression on the standard dose. Imatinib Cmin monitoring may help to manage the patients with standard dose-resistant GIST that may require dose escalation.

To evaluate the efficacy of nilotinib in patients with advanced gastrointestinal stromal tumors (GISTs) resistant or intolerant to both imatinib and sunitinib and to explore the potential relationship between nilotinib pharmacokinetics and clinical outcomes. We analyzed the efficacy, tolerability and pharmacokinetic parameters of nilotinib (400 mg twice daily) in 17 GIST patients with histories of prior gastrointestinal surgery. Median patient age was 59 years (range, 35–71 years), 14 of 17 patients (82.4%) were male, and mean body weight was 59.4 kg. Of the 17 patients, 2 (11.8%) had partial responses (PR), 10 (58.8%) had stable disease (SD), and 5 (29.4%) had progressive disease (PD), with a clinical benefit rate (CR + PR + SD) at 24 weeks of 47.0%. Median progression-free survival (PFS) and overall survival (OS) were 23.6 weeks (95% confidence interval [CI] 0.0–50.6 weeks) and 74.0 weeks (95% CI 27.4–120.6 weeks), respectively. Most observed adverse events were mild (grade 1, 41.2%; grade 2, 52.9%), with no grade 3/4 events. Pharmacokinetic parameters of nilotinib were as follows: C max of 1,754 ± 970 μg/L, T1/2 of 13.4 ± 8.94 h and AUC 0–12 h of 14,190 ± 6,853 h μg/L. The AUC 0–12 h of nilotinib was significantly lower in the 4 patients with prior major (total or subtotal) gastrectomy than in the other 13 patients (8,526 ± 7,869 h μg/L vs. 15,930 ± 5,759 h μg/L, P = 0.014). Of the 4 gastrectomized patients, two (50%) showed markedly decreased nilotinib exposure (AUC 0–12 h of 1,914 and 3,194 h μg/L) and rapid disease progression (PFS of 4.6 and 7.1 weeks). Nilotinib was active and safe in patients with advanced GIST resistant to both imatinib and sunitinib. Major gastrectomy decreased the bioavailability of nilotinib and, in some patients, lowered its clinical activity.

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.

Imatinib is a substrate of drug transporters and metabolizing enzymes, including members of the cytochrome P450 (CYP) system. Differences in imatinib pharmacokinetics among individuals might be influenced by genetic polymorphisms and be associated with variable clinical imatinib efficacy. This study sought to test how genetic polymorphisms can affect the clinical efficacy of imatinib and its blood levels in GIST patients. A total of 209 GIST patients who had received imatinib 400 mg daily were genotyped for six single-nucleotide polymorphisms in three genes (CYP3A5 6986A>G; ABCB1 1236C>T, 2677G>A/T, and 3435C>T; and ABCG2 34G>A and 421C>A) via blood samples. Progression-free survival (PFS) and imatinib plasma trough levels were evaluated and compared according to genotypes. With a median follow-up of 39.6 months (range 16.7–97.5 months), the estimated 5-year PFS rate was 67.5 % (95 % CI 59.9–75.1). Among the CYP3A5, ABCB1, and ABCG2 genotypes, ABCG2 421C>A was associated with PFS. The 5-year PFS rate in patients with the AA variant of ABCG2 421C>A (92.3 %; 95 % CI 77.8–100.0) was significantly superior to that of patients with CC/CA genotypes (65.0 %; 95 % CI 56.9–73.1; p = 0.047). For the imatinib trough levels, there were no statistically significant differences when comparing polymorphisms among all genotypes, even after adjusting for clinical factors, including sex, age, body surface area, hemoglobin, albumin, and creatinine clearance. The ABCG2 421C>A genetic variation could influence clinical efficacy in terms of PFS in patients with advanced GIST undergoing imatinib therapy.

10085 Background: Although escalated dose (ED, 800 mg daily) of imatinib (IM) is one of important options in resistant gastrointestinal stromal tumors (GIST) on standard dose (SD, 400mg daily), there has been lack of pharmacokinetic data at ED. We therefore explore the potential relationship between IM plasma trough level (Cmin), and clinical outcomes or toxicities in patients treated with ED. Methods: Between 2008 and 2011, steady-state IM Cmin was measured in 66 patients with GIST who received ED of IM using liquid chromatography-tandem mass spectrometry. Percent change from IM Cmin at SD was calculated in 43 patients that both SD and ED IM Cmin were measured. The association with efficacy and toxicity was analyzed with grouping patients into quartiles according to IM Cmin at ED and percent change. Results: Median age was 59 years (range, 36–77) and 40 patients were male. KIT exon 11 and 9 mutation was detected in 32 and 18 patients, respectively. At ED, partial response was achieved in 4 patients (6%) and stable disease was in 32 patients (48%). The median progression-free survival was 4.2 months (95% CI, 1.8-6.6) and overall survival was 38.5 months (95% CI, 7.6-69.3). The mean (± standard deviation) IM Cmin at ED was 3552 (± 1540) ng/mL. IM Cmin was significantly increased after dose escalation (p min at SD was 162% (± 100). Body surface area (p=0.01), hemoglobin (p=0.001), and neutrophil count (p=0.006) were significant covariates for IM Cmin at ED in multivariate analysis. The group of quartiles of IM Cmin at ED and percent change was not associated with response and survival outcomes. However, grade 3-4 hematologic or grade 2-4 non-hematologic toxicity was observed in 9% (1/11) of patients in the lowest percent change quartile (Q1, min was not associated with response and survival. Clinically significant toxicity following dose escalation was correlated with percent change of IM Cmin, rather than absolute values. Monitoring of IM Cmin might help to predict or manage ED of IM induced toxicity.

306 Background: Pharmacokinetic study in patients with gastrointestinal stromal tumors (GISTs) suggested that plasma concentrations of imatinib decrease following long-term exposure. We therefore measured changes in imatinib plasma trough levels (Cmin) after long-term exposure. Methods: Between November 2009 and May 2010, follow-up (FU) imatinib Cmin was measured in 65 patients who received the same dose of imatinib for at least 9 months after a previous baseline (BL) measurement. Total 244 blood samples were obtained (127 at BL and 117 at FU) and plasma level was measured by liquid chromatography-tandem mass spectrometry. Results: Median patient age was 54 years (range, 28–76 years) and 42 (64.6%) patients were male. Sixty-one (93.8%) patients were treated with 400 mg/day imatinib and 4 (6.2%) with 300 mg/day. The median interval from initiation of imatinib to BL test was 6.4 months (range, 0.5–66.6 months), and the median interval between BL and FU test was 13.1 months (range, 9.6–18.4 months). The mean ± standard deviation imatinib Cmin was significantly higher at FU than at BL (1442 ± 693 ng/mL vs 1221 ± 624 ng/mL, pmin and that of albumin (r=-0.397, p=0.001). In per-sample analysis, imatinib Cmin was significantly correlated with age, hemoglobin, albumin, creatinine clearance, previous major gastrectomy and time between initiation of imatinib and plasma level tests. Conclusions: Steady-state imatinib Cmin did not decrease but remained stable in most GIST patients during long-term treatment. Changes in imatinib Cmin were associated with changes in albumin concentration. Monitoring of imatinib Cmin only for concerns about time-dependent decreases in imatinib exposure is not necessary. [Table: see text]

10016 Background: Nilotinib, a novel tyrosine kinase inhibitor (TKI), has been suggested to be active for advanced gastrointestinal stromal tumors (GISTs). Drug-food interaction studies on the bioavailability of nilotinib suggest the importance of the stomach in nilotinib absorption. Methods: We analyzed the efficacy, tolerability and PK parameters of nilotinib (400 mg twice daily) in 17 consecutive patients who had advanced GISTs resistant to both imatinib and sunitinib treated with nilotinib in Compassionate Use Program at the Asan Medical Center, Seoul, Korea between May 2007 and October 2009. Results: The median age was 59 years (range 35-71). Most of the patients were male (82.4%). Mean body weight was 59.4 kg. Among the total 17 patients, one (5.9%) had partial response (PR), 12 (70.6%) had stable disease (SD), and 4 (23.5%) had progressive disease (PD), according to RECIST criteria. Clinical benefit rate (CR+PR+SD) at 24 weeks was 47.1%. The median progression-free survival (PFS) and overall surviv...