Therapy for patients with metastatic CaP typically involves androgen deprivation. Although this therapy is effective initially, most patients will ultimately develop castration-resistant CaP. At that point, the current standard of care is chemotherapy with docetaxel, which offers an incremental survival advantage for these patients. This underscores the need for additional therapies in treating patients with metastatic CaP. A potential avenue for improving outcomes for patients with metastatic CaP involves the use of small molecule inhibitors of signalling pathways either alone or in combination (McCarty, 2004
). SFK inhibitors fall into this category and there are several of these being evaluated in both the preclinical and clinical setting. As src levels are increased in advanced CaP and possibly even more so in castration-resistant disease, we sought to investigate the efficacy of a src inhibitor, dasatinib, in treating CaP growth in bone.
We have demonstrated that dasatinib inhibits proliferation of C4-2B CaP cells in vitro.
Most importantly, we showed that PSA production is not affected in the absence of growth inhibition. This point is critical to our in vivo
studies as it allows for PSA levels to be used as a surrogate marker for tumour growth in bone in response to this therapy. Also of interest, despite recent literature demonstrating the importance of the SFK member, lyn, in regulating proliferation of PC-3 CaP cells (Park et al, 2008
); we show that lyn phosphorylation is unaffected in C4-2B cells on treatment with dasatinib. Thus implying that lyn inactivation is not involved in the inhibition of proliferation by dasatinib in these cells.
Our in vivo studies have indicated that dasatinib decreased tumour growth in the bone environment. First, we used serum PSA levels to evaluate effects on tumour growth, and our data showed that dasatinib caused decreases in serum PSA. This effect is important, as PSA levels are the major surrogate marker of anti-tumour response in tumours growing intratibially while under treatment. Although it is generally accepted that in primary disease PSA correlates with tumour volume, it is also noted from preclinical and clinical data that in advanced castration-resistant CaP, changes in serum PSA levels do not necessarily reflect changes in tumour volume. We believe, however, that our in vitro results allow us to extrapolate that the decreases in serum PSA levels seen in mice bearing intratibial tumours are synonymous with decreases in tumour volume. To further confirm dasatinib inhibition of tumour growth by means other than serum PSA levels, we examined the weight of the tibiae with tumour. The decreases in weight of the tibiae observed in those groups receiving either docetaxel, dasatinib or a combination compared to the control group help to substantiate the claim that treatment resulted in decreased tumour burden. This may be especially accurate for dasatinib (alone or in combination)-treated tibiae where decreases in tibiae weight were seen despite increases in BMD, which implies that the weight loss is related to decreases in tumour volume and not bone destruction. The increased BMD of the groups receiving dasatinib alone or in combination may also be a result of decreased tumour growth and not solely related to the effect that dasatinib has on regulating osteoclast activity. Altogether, these data indicate that dasatinib has a significant cytotoxic effect on CaP cells in the bone environment.
An issue to mention is the lack of significant effects seen in the docetaxel monotherapy group. Our previous work with docetaxel had shown that the dose of 5
to be an efficacious dose, albeit those studies were performed on a different cell line. In this study, the docetaxel when used alone did slow tumour growth by ~30%, but these results did not reach significance. However, when docetaxel was used in combination with dasatinib, the dose resulted in significant decreases in PSA levels beyond those seen with the administration of dasatinib alone. We have previously seen this increased efficacy with the use of other pharmaceuticals in combination with docetaxel over either agent alone (Brubaker et al, 2006
; Morgan et al, 2008
). In addition, src inhibition has been reported to increase sensitivity of other advanced cancers to several already used chemotherapies and specifically to docetaxel resistance (Yezhelyev et al, 2004
; Han et al, 2006
). This phenomenon may have important implications for patient care as of the efficacy of docetaxel therapy may be enhanced with the addition of dasatinib. Nonetheless, final conclusions regarding effects of dasatinib in this regard will require additional clinical investigations.
Src is an important player in forming the cell–cell junctions necessary for bone resorption to take place, and it has been clearly shown that inhibition of src activity decreases bone osteolysis. However, our results are the first to show that dasatinib treatment results in increases in BMD of normal bone as well as increases in BMD of bone harbouring an osteolytic tumour. The ability of dasatinib to increase BMD may prove to be beneficial to patients with CaP where skeletal related events (SRE; ie, pathologic fractures, spinal cord compression) and bone pain are a significant source of morbidity. Additionally, even patients without bone metastases suffer an increased risk of osteoporotic fractures from the effects of androgen deprivation. Agents that can cause increases in BMD have been investigated in metastatic prostate cancer. The bisphosphonate, zoledronic acid, has been shown to increase time to first SRE as well as reduced the number of SREs in patients with castration-resistant metastatic prostate cancer (Saad et al, 2004
). Inhibition of RANKL has also been shown to increase BMD in experimental prostate cancer bone metastasis models (Ignatoski et al, 2008
) and inhibitors (AMG162, denosumab) are currently in clinical trials. These agents have been shown to affect tumour growth mainly via indirect mechanisms by inhibiting bone lysis. In contrast, decreasing activity of src has been shown not only to inhibit osteolysis but also directly inhibit tumour growth. Therefore, the use of compounds such as dasatinib has the potential to provide pronounced benefit for metastatic prostate cancer patients.
In summary, our results indicate that dasatinib inhibits the growth of CaP in bone, and these inhibitory effects are increased in combination with docetaxel. Additionally, our study demonstrates decrease in osteolysis in dasatinib-treated animals, both in the tumour as well as normal bone. Our results suggest that dasatinib alone or in combination should be further investigated in a clinical setting for patients with metastatic CaP, and these studies are currently ongoing.