Phase I pharmacokinetic and pharmacodynamic study of the first-in-class spliceosome inhibitor E7107 in patients with advanced solid tumors

Purpose: To assess the safety, tolerability, pharmacokinetics, pharmacodynamics, and clinical activity of E7107 administered as 5-minute bolus infusions on days 1, 8, and 15 in a 28-day schedule. Experimental Design: Patients with solid tumors refractory to standard therapies or with no standard treatment available were enrolled. Dose levels of 0.6 to 4.5 mg/m2 were explored. Results: Forty patients [24M/16F, median age 61 years (45-79)] were enrolled. At 4.5 mg/m2, doselimiting toxicity (DLT) consisted of grade 3 diarrhea, nausea, and vomiting and grade 4 diarrhea, respectively, in two patients. At 4.0 mg/m2, DLT (grade 3 nausea, vomiting, and abdominal cramps) was observed in one patient. Frequently occurring side effects were mainly gastrointestinal. After drug discontinuation at 4.0 mg/m 2, one patient experienced reversible grade 4 blurred vision. The maximum tolerated dose (MTD) is 4.0 mg/m2. No complete or partial responses during treatment were observed; one patient at 4.0 mg/m2 had a confirmed partial response after drug discontinuation. Pharmacokinetic analysis revealed a large volume of distribution, high systemic clearance, and a plasma elimination halflife of 5.3 to 15.1 hours. Overall drug exposure increased in a dose-dependent manner. At the MTD, mRNA levels of selected target genes monitored in peripheral blood mononuclear cells showed a reversible 15- to 25-fold decrease, whereas unspliced pre-mRNA levels of DNAJB1 and EIF4A1 showed a reversible 10- to 25-fold increase. Conclusion: The MTD for E7107 using this schedule is 4.0 mg/m2. Pharmacokinetics is dose-dependent and reproducible within patients. Pharmacodynamic analysis revealed dose-dependent reversible inhibition of pre-mRNA processing of target genes, confirming proof-of-principle activity of E7107

Cancer-Associated SF3B1 Hotspot Mutations Induce Cryptic 3′ Splice Site Selection through Use of a Different Branch Point

Recurrent mutations in the spliceosome are observed in several human cancers, but their functional and therapeutic significance remains elusive. SF3B1, the most frequently mutated component of the spliceosome in cancer, is involved in the recognition of the branch point sequence (BPS) during selection of the 3′ splice site (ss) in RNA splicing. Here, we report that common and tumor-specific splicing aberrations are induced by SF3B1 mutations and establish aberrant 3′ ss selection as the most frequent splicing defect. Strikingly, mutant SF3B1 utilizes a BPS that differs from that used by wild-type SF3B1 and requires the canonical 3′ ss to enable aberrant splicing during the second step. Approximately 50% of the aberrantly spliced mRNAs are subjected to nonsense-mediated decay resulting in downregulation of gene and protein expression. These findings ascribe functional significance to the consequences of SF3B1 mutations in cancer

H3B-8800, an orally available small-molecule splicing modulator, induces lethality in spliceosome-mutant cancers

Genomic analyses of cancer have identified recurrent point mutations in the RNA splicing factor-encoding genes SF3B1, U2AF1, and SRSF2 that confer an alteration of function(1-6). Cancer cells bearing these mutations are preferentially dependent on wild-type (WT) spliceosome function(7-11), but clinically relevant means to therapeutically target the spliceosome do not currently exist. Here we describe an orally available modulator of the SF3b complex, H3B-8800, which potently and preferentially kills spliceosome-mutant epithelial and hematologic tumor cells. These killing effects of H3B-8800 are due to its direct interaction with the SF3b complex, as evidenced by loss of H3B-8800 activity in drug-resistant cells bearing mutations in genes encoding SF3b components. Although H3B-8800 modulates WT and mutant spliceosome activity, the preferential killing of spliceosome-mutant cells is due to retention of short, GC-rich introns, which are enriched for genes encoding spliceosome components. These data demonstrate the therapeutic potential of splicing modulation in spliceosome-mutant cancers