Probes for CSF1

 BACKGROUND:
 Expression of the colony-stimulating factor 1 (CSF1) gene is elevated in most tenosynovial giant-cell tumors. This observation has led to the discovery and clinical development of therapy targeting the CSF1 receptor (CSF1R).
 METHODS:
 Using x-ray co-crystallography to guide our drug-discovery research, we generated a potent, selective CSF1R inhibitor, PLX3397, that traps the kinase in the autoinhibited conformation. We then conducted a multicenter, phase 1 trial in two parts to analyze this compound. In the first part, we evaluated escalations in the dose of PLX3397 that was administered orally in patients with solid tumors (dose-escalation study). In the second part, we evaluated PLX3397 at the chosen phase 2 dose in an extension cohort of patients with tenosynovial giant-cell tumors (extension study). Pharmacokinetic and tumor responses in the enrolled patients were assessed, and CSF1 in situ hybridization was performed to confirm the mechanism of action of PLX3397 and that the pattern of CSF1 expression was consistent with the pathological features of tenosynovial giant-cell tumor.
 RESULTS:
 A total of 41 patients were enrolled in the dose-escalation study, and an additional 23 patients were enrolled in the extension study. The chosen phase 2 dose of PLX3397 was 1000 mg per day. In the extension study, 12 patients with tenosynovial giant-cell tumors had a partial response and 7 patients had stable disease. Responses usually occurred within the first 4 months of treatment, and the median duration of response exceeded 8 months. The most common adverse events included fatigue, change in hair color, nausea, dysgeusia, and periorbital edema; adverse events rarely led to discontinuation of treatment.
 CONCLUSIONS:
 Treatment of tenosynovial giant-cell tumors with PLX3397 resulted in a prolonged regression in tumor volume in most patients. (Funded by Plexxikon; ClinicalTrials.gov number, NCT01004861.).

Abstract

INTRODUCTION:

Localized- and diffuse-type tenosynovial giant cell tumours (TGCT) are regarded different clinical and radiological TGCT-types. However, genetically and histopathologically they seem indistinguishable. We aimed to correlate CSF1-expression and CSF1-rearrangement with the biological behaviour of different TGCT-types with clinical outcome (recurrence).

METHODS:

Along a continuum of extremes, therapy naïve knee TGCT patients with >3 year follow-up, mean age 43(range 6-71)years and 56% female were selected. Nine localized-(two recurrences), 16 diffuse-type(nine recurrences) and four synovitis as control were included. Rearrangement of the CSF1-locus was evaluated with split-apart Fluorescence In Situ Hybridization (FISH) probes. Regions were selected to score after identifying CSF1-expressing regions, using mRNA ISH with the help of digital correlative microscopy. CSF1-rearrangement was considered positive in samples containing >2 split signals/100 nuclei.

RESULTS:

Irrespective of TGCT-subtype, all cases showed CSF1-expression and in 76% CSF1-rearrangement was detected. Quantification of CSF1-expressing cells was not informative, due to the extensive intra tumour heterogeneity. Of the four synovitis cases, two also showed CSF1-expression, without CSF1-rearrangement. No correlation between CSF1-expression or rearrangement with clinical subtype and local recurrence was detected. Both localized- and diffuse-TGCT cases showed a scattered distribution in the tissue of CSF1-expressing cells.

CONCLUSION:

In diagnosing TGCT, CSF1 mRNA-ISH in combination with CSF1 split-apart FISH; using digital correlative microscopy, is an auxiliary diagnostic tool to identify rarely occurring neoplastic cells. This combined approach allowed us to detect CSF1-rearrangement in 76% of the TGCT-cases. Neither CSF1-expression nor presence of CSF1-rearrangement could be associated with the difference in biological behaviour of TGCT.