Prostate cancer and bone: a unique relationship with multiple opportunities for targeted therapy

Clinical use of bone-targeting radiopharmaceuticals with focus on alpha-emitters

Various single or multi-modality therapeutic options are available to treat pain of bone metastasis in patients with prostate cancer. Different radionuclides that emit β-rays such as ¹⁵³Samarium and ⁸⁹Strontium and achieve palliation are commercially available. In contrast to β-emitters, ²²³Radium as a α-emitter has a short path-length. The advantage of the α-emitter is thus a highly localized biological effect that is caused by radiation induced DNA double-strand breaks and subsequent cell killing and/or limited effectiveness of cellular repair mechanisms. Due to the limited range of the α-particles the bone surface to red bone marrow dose ratio is also lower for ²²³Radium which is expressed in a lower myelotoxicity. The α emitter ²²³Radium dichloride is the first radiopharmaceutical that significantly prolongs life in castrate resistant prostate cancer patients with wide-spread bone metastatic disease. In a phase III, randomized, double-blind, placebo-controlled study 921 patients with castration-resistant prostate cancer and bone metastases were randomly assigned. The analysis confirmed the ²²³Radium survival benefit compared to the placebo (median, 14.9 mo vs 11.3 mo; P < 0.001). In addition, the treatment results in pain palliation and thus, improved quality of life and a delay of skeletal related events. At the same time the toxicity profile of ²²³Radium was favourable. Since May 2013, ²²³Radium dichloride (Xofigo^®) is approved by the US Food and Drug Administration.

Magnetic resonance imaging of the axial skeleton enables objective measurement of tumor response on prostate cancer bone metastases

BACKGROUND

There is currently no technique to image quantitatively bone metastases. Here, we assessed the value of MRI of the axial skeleton (AS-MRI) as a single step technique to quantify bone metastases and measure tumor response.

METHODS

AS-MRI was performed in 38 patients before receiving chemotherapy for metastatic HRPCa, in addition to PSA, computed tomography of the thorax, abdomen, and pelvis [CT-TAP]; and Tc-99m bone scintigraphy. A second AS-MRI was performed in 20 patients who completed 6 months of chemotherapy. Evaluation of tumor response was performed using RECIST.

RESULTS

Only 11 patients (29%) had RECIST measurable metastases in soft-tissues or lymph nodes on baseline CT-TAP. AS-MRI identified a diffuse infiltration of the bone marrow in 8 patients and focal measurable metastatic lesions in 25 patients (65%), therefore, doubling the proportion of patients with measurable lesions. Transposing RECIST on AS-MRI in 20 patients who completed 6 months of treatment, allows the accurate estimation of complete response (n = 2), partial response (n = 2), stable disease (n = 5), or tumor progression (n = 11), as it is done using CT-TAP in soft tissue solid metastases.

CONCLUSIONS

MRI of axial skeleton enables precise measurement and follow-up of bone metastases as it is for other soft-tissue metastasis.