Moving Forward One Notch at a Time

In the articles that accompany this editorial, Tolcher et al and Krop et al report the results of two phase I studies that evaluated the Notch inhibitors RO4929097 from Roche and MK-0752 from Merck. Both drugs inhibit -secretase, which regulates the Notch pathway. Notch signaling is an evolutionarily conserved pathway that is important in the normal development of multiple organ systems. The activation of the Notch pathway occurs when specific ligands, Delta-like and Jagged, bind to four related transmembrane receptors, Notch1 through Notch4. This binding activates the -secretase protein complex, which cleaves the Notch receptor in the transmembrane domain to release the cytoplasmic portion, the Notch1 intracellular domain or cleaved notch. After translocating into the nucleus, the Notch1 intracellular domain binds to cofactors and transcriptionally activates multiple downstream targets. However, if aberrantly activated, the Notch pathway may contribute to tumorigenesis by inducing prosurvival targets, such as phosphatidylinositol 3-kinase (PI3K)/Akt and survivin, and growth-promoting targets, such as cyclin D1 and c-myc. Gamma secretase inhibitors (GSIs) were originally targeted for patients with Alzheimer’s disease, in which the amyloid precursor protein is cleaved by -secretase to generate a neurotoxic amyloid -protein. Clinical trials with these drugs in Alzheimer’s disease were generally negative, perhaps in part as a result of poor or variable CNS penetration. The first attempt to evaluate GSIs for cancer therapy was in the setting of human T-lineage acute lymphoblastic leukemia (T-ALL); the majority of patients with T-ALL have activating mutations in Notch1, and GSIs induced cell cycle arrest in T-ALL cell lines that expressed wildtype or mutant Notch. This set the ideal stage for the clinical development of GSIs in this disease. The first phase I clinical trial of MK-0752, which involved 10 patients with relapsed or refractory T-ALL, six of whom had activating Notch1 mutations, proved disappointing. The orally administered daily dose (300 mg/m per day) had severe toxicities (diarrhea, fatigue, and cough), and patients were on the trial for a median of 11 days. The dose and daily schedule were clearly too toxic. Evaluation of Notch1 levels in peripheral blood mononuclear cells did not correlate with gain-of-function Notch1 mutations or clinical benefit. The reason for the resistance to MK-0752 in T-ALL remains unclear. Recently, gene profiling and mutational analysis have shown that loss of phosphatase and tensin homolog and upregulation of the PI3K/Akt pathway is a common event in T-ALL and is associated with resistance to GSIs, thus laying the groundwork for rational combination studies. It is in this background that these two phase I studies of RO4929097and MK-0752 were initiated in adults with advanced solid tumors. The primary aims of the two trials were to determine a maximum-tolerated dose (MTD) for phase II dosing, assess safety, examine potential clinical activity, and examine pharmacokinetic (PK) and pharmacodynamic (PD) end points. By phase I standards, these two studies are quite large: 110 patients were treated with RO4929097 and 103 patients were treated with MK-0752. Why did it take so many patients to define the MTD? Importantly, are we any closer to understanding the biology of targeting the Notch pathway? In the article by Krop et al, MK-0752 was reinvestigated in solid tumors under three schedules and at flat doses: schedule A with continuous, once-per-day dosing; schedule B with dosing on 3 days of 7; and schedule C with once-per-week dosing. Both schedules A and B evaluated two cohorts before dose-limiting diarrhea occurred with schedule A and unexpected dose-limiting fatigue occurred with schedule B. Dose expansions at the lower dose of 450 mg on both schedules indicated unacceptable toxicity; therefore, a weekly dosing schedule was pursued. However, by the time it was decided that neither schedule A nor B was worth pursuing, a total of 38 patients had already been treated in the expansion cohort. It is unclear why schedule A or B was reinvestigated when the dose (similar to allometric dose) and the schedules (daily, or 3 days on/4 days off) were clearly too toxic in the previously reported MK-0752 study in patients with T-ALL. Not surprisingly, identical toxicities were reported in both studies. On schedule C (once-per-week dosing), 65 patients were treated in eight cohorts. It is not clear why some cohorts included eight patients or why two dose expansions were investigated; one at the MTD dose of 3,200 mg weekly, and another at 1,800 mg weekly. It is not unusual for phase I studies to expand cohorts to further confirm drug tolerability. However, in this study, there is no explanation provided for the additional patients who were treated in the dose escalations or expansion cohorts. Importantly, MK-0752 has a time from dosing to maximum plasma concentration of 8 hours and a half-life of 15 hours. A single weekly dose of MK-0752 would not achieve steady-state levels. However, the investigators do show a correlation between a nine-gene Notch signature score and pharmacologic exposure of MK-0752 in individual patients, thus confirming target inhibition. This PD biomarker of Notch inhibition was first derived from a healthy volunteer study and included changes in Notch transcriptional expression in preand post-treatment hair follicles at 8 hours. In addition to Notch, PI3K and cell proliferation signals were also suppressed. However at 48 hours, there was a compensatory upregulation of Wnt and other receptor tyrosine kinases. The limitation of this study is that the PD biomarker represents a single and early time point (8 hours) in a weekly schedule in which the maximum systemic exposure of the drug would not be expected to be greater than JOURNAL OF CLINICAL ONCOLOGY E D I T O R I A L VOLUME 30 NUMBER 19 JULY 1 2012