Pharmacodynamic response in a phase 1 clinical study of the potent Hedgehog pathway antagonist GDC-0449 using a GLI1 qRT-PCR assay

B29

The Hedgehog (Hh) signaling pathway is an important regulator of organ patterning during embryonic development and is restricted to a few post-embryonic tissues, including hair follicles. However, the Hh pathway has been shown to be re-activated in a subset of tumors, including basal cell carcinomas (BCC) and medulloblastomas, as a result of somatic mutations in genes encoding the cell surface receptors Patched-1 (PTCH1) and/or Smoothened (SMO). Hh pathway activation results in the activation of the GLI transcription factors, which induce the transcriptional upregulation of hedgehog target genes, including GLI1 itself. We have developed a well-characterized qRT-PCR assay from hair pluck and skin punch biopsy (SPB) samples for normalized GLI1 that is greater than 95 percent efficient, is quantitative over a 61-fold range of input RNA, and has an intra-donor variability of 1.24-fold, which easily allows for the measurement of a two-fold or higher change in normalized GLI1 expression between samples (the minimum down-modulation of GLI1 observed in pre-clinical efficacy models).
 GDC-0449 is a potent antagonist of the Hh pathway tested in a phase 1 clinical study to evaluate its safety, tolerability, MTD, PK, and pharmacodynamics (PD) in patients with solid tumors. Hair pluck and SPB samples were collected from patients for PD analysis, assessing changes in expression levels of GLI1 mRNA, prior to dosing and after eight and 21 days of continuous daily dosing in the dose escalation cohorts (150, 270 and 540 mg), a safety cohort and in a BCC cohort (both 150 mg). PD samples were collected at the day eight timepoint only for the 150 mg GDC-0449 DE cohort. Plasma GDC-0449 concentrations coincident with PD sample collection times were all above the targeted trough concentration of 5uM, based on pre-clinical murine efficacy models.
 Whereas measured post-dose GLI1 expression from hair pluck samples collected in the phase 1 study showed a variable PD response compared to pre-dose GLI1 mRNA levels, a 2-14-fold decrease in GLI1 expression was observed in SPB samples from 18 of 23 (78%) patients, across all dose cohorts. Similar intra-patient changes in GLI1 expression were observed in SPB samples collected after 8 and 21 days of daily dosing. These PD data, utilizing a well-characterized qRT-PCR assay, provide evidence that GDC-0449 is modulating the hedgehog pathway in relevant tissue. These data supported the selection of a phase 2 dose.