Unexpectedly high affinity of a novel histamine H(3) receptor antagonist, GSK239512, in vivo in human brain, determined using PET

BACKGROUND AND PURPOSE

This study aimed to investigate the relationship between the plasma concentration (PK) of the novel histamine H3 receptor antagonist, GSK239512, and the brain occupancy of H(3) receptors (RO) in healthy human volunteers.

EXPERIMENTAL APPROACH

PET scans were obtained after i.v. administration of the H(3) -specific radioligand [(11) C]GSK189254. Each subject was scanned before and after single oral doses of GSK239512, at 4 and 24 h after dose. PET data were analysed by compartmental analysis, and regional RO estimates were obtained by graphical analysis of changes in the total volumes of distribution of the radioligand, followed by a correction for occupancy by the high affinity radioligand. The PK/RO relationship was analysed by a population-modelling approach, using the average PK of GSK239512 during each scan.

KEY RESULTS

Following administration of GSK239512, there was a reduction in the brain uptake of [(11) C]GSK189254 in all regions, including cerebellum. RO at 4 h was higher than at 24 h, and the PK/RO model estimated a PK associated with 50% of RO of 0.0068 ng·mL(-1) . This corresponds to a free concentration of 4.50 × 10(-12 ) M (pK = 11.3).

CONCLUSIONS AND IMPLICATIONS

The affinity of GSK239512 for brain H3 receptors in humans in vivo is much higher than that expected from studies in vitro, and higher than that observed in PET studies in pigs. The study illustrates the utility of carrying out PET studies in humans early in drug development, providing accurate quantification of GSK239512 RO in vivo as a function of time and dose.

Abstract P4-05-05: Stromal Response to 14-Day Preoperative Therapy in Postmenopausal Oestrogen Receptor Positive Breast Cancer

Background: Stromal-epithelial interaction is a key factor in tumour progression. Cancer-associated fibroblasts (CAFs) and macrophage infiltration have been associated with early relapse in breast cancer. Bisphosphonates are effective inhibitors of osteoclast activation in metastatic breast cancer but also have a general inhibitory effect on breast cancer progression. In order to monitor a potential tumour stromal response in breast cancer during treatment with an aromatase inhibitor and a bisphosphonate we analysed pre-and post-treatment samples from a neoadjuvant window study and focused on the presence of macrophages and CAFs.

Materials and methods: Tissue microarrays (TMAs) from surgical samples and pre-operative core biopsies were immunohistochemically stained for aSMA (CAF marker), CD68 (macrophages) and epithelial proliferation (Ki67). In order to validate if the presence of macrophages and aSMA could be monitored by the TMA approach, we initially analysed a screening cohort of 144 breast cancer samples. We then studied pre-and post-treatment samples from 110 postmenopausal ER-positive invasive breast cancer patients randomised to receive 14 days of preoperative treatment (placebo, Letrozole, or Letrozole plus Zoledronate). Results: In the screening cohort, we observed significant links between aSMA positive fibroblasts and disease recurrence as well as between CD68 positive macrophages and tumour size, grade, lymph node positivity and recurrence. This validated the use of TMAs for stromal analyses and furthersupported a link with key tumour biological events. In both treatment arms, there was a significant drop in absolute Ki67 value compared to placebo (-9.3% Letrozole and -13.1% combination reduction versus 1% increase, P<0.001). Post-treatment CD68 (median 35, range 3 to 117) was significantly linked to a Ki67 drop (p=0.045). Interestingly, this effect was mainly observed in the combination treatment group (p=0.002). aSMA expression was unaffected during treatment in 52%, increased in 35% and decreased in 13% of cases. Patients with aSMA reduction post treatment had a larger Ki67 fall compared to patients with increase or no change in aSMA (p=0.007).

Conclusion: Short term treatment response in the epithelial component of cancers was paralleled by specific responses in the tumour stromal component. These novel findings suggest that bisphosphonates and aromatase inhibitors have major effects on tumour stroma in vivo which might augment their inhibitory effect on tumour progression.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P4-05-05.