Heat shock protein 90 as a drug target: some like it hot

Heat shock protein 90 (HSP90) is a ubiquitously expressed chaperone that is involved in the posttranslational folding and stability of proteins. Inhibition at the NH(2)-terminal ATP-binding site leads to the degradation of client proteins by the ubiquitin proteasome pathway. Inhibition of HSP90 leads to the degradation of known oncogenes, such as ERB-B2, BRAF, and BCR-ABL, leading to the combinatorial blockade of multiple signal transduction pathways, such as the RAS-RAF-mitogen-activated protein/extracellular signal-regulated kinase kinase-extracellular signal-regulated kinase and phosphatidylinositol 3-kinase pathways. Multiple structurally diverse HSP90 inhibitors are undergoing early clinical evaluation. The clinical focus of these drugs should be solid tumors, such as breast, prostate, and lung cancers, along with malignant melanoma, in addition to hematologic malignancies, such as chronic myeloid leukemia and multiple myeloma. HSP90 inhibitors can be used as single agents or in combination with other targeted treatments or conventional forms of treatment such as chemotherapy and radiotherapy. Clinical trials evaluating efficacy of these agents should include innovative designs to capture cytostasis evidenced by clinical nonprogression and enrichment of patient populations by molecular characterization. The results of clinical trials evaluating the efficacy of drugs targeting this exciting target are awaited.

Evaluation of Plasma Insulin-like Growth Factor Binding Protein 2 and Her-2 Extracellular Domain as Biomarkers for 17-Allylamino-17-Demethoxygeldanamycin Treatment of Adult Patients with Advanced Solid Tumors

PURPOSE

Interaction of 17-allylamino-17-demethoxygeldanamycin (17-AAG) with heat shock protein 90 results in proteasomal degradation of many proteins, including Her-2-neu, with subsequent decreased expression of insulin-like growth factor binding protein-2 (IGFBP-2). Concentrations of both IGFBP-2 and Her-2 extracellular domain (Her-2 ECD) in sera of mice bearing BT474 human breast cancer xenografts decrease after 17-AAG treatment. We investigated whether this phenomenon occurred in patients.

MATERIALS AND METHODS

Eight to 15 plasma samples were obtained between 0 and 72 h from 27 patients treated with single-agent 17-AAG at doses between 10 and 307 mg/m(2) and 18 patients treated with 17-AAG at doses between 220 and 450 mg/m(2) combined with 70 to 75 mg/m(2) of docetaxel. Pretreatment plasma samples were also obtained from 12 healthy volunteers. Plasma IGFBP-2 and Her-2 ECD concentrations were quantitated by ELISA.

RESULTS

Pretreatment plasma IGFBP-2 concentrations in patients (171 +/- 116 ng/mL) were 2-fold higher than those in healthy volunteers (85 +/- 44 ng/mL; P < 0.05). Following 17-AAG treatment, there were no consistent dose-dependent or time-dependent changes in plasma IGFBP-2 and Her-2 ECD concentrations. IGFBP-2 concentrations decreased by >or=40% in 8 patients, increased 2- to 5-fold in 8 patients, and remained essentially unchanged in 29 patients. Her-2 ECD concentrations decreased by >or=40% in 10 patients, increased 1.5- to 5-fold in 2 patients, and remained essentially unchanged in 25 patients.

CONCLUSIONS

As previously reported, IGFBP-2 concentrations in plasma of cancer patients are significantly higher than those in healthy volunteers. In contrast to a mouse model, 17-AAG treatment was not consistently associated with decreases in IGFBP-2 or Her-2 ECD concentrations in patient plasma.