**2.2 Physiopathology**

GCTB physiopathology is not entirely understood, but there is compelling evidence that RANKL overexpression by mononuclear stromal cells plays a key role and elicits transformation of monocytic pre-osteoclast to osteoclast cells, ultimately resulting in osteolysis observed in these tumors [22, 41–43] (**Figure 2**). Accordingly, preclinical models have shown that OPG, a soluble decoy receptor for RANKL, inhibits monocyte activation and osteoclast differentiation [44].

#### **Figure 2.**

*Simplified scheme of GCTB physiopathology. BMP-2, bone morphogenetic protein 2; CCR2, C-C chemokine receptor type 2; CXCR4, C-X-C chemokine receptor type 4; DC-STAMP, dendritic cell-specific transmembrane protein; IL-1, interleukin 1; MCP-1, monocyte chemoattractant protein-1; M-CSF, macrophage colony-stimulating factor; MIP-1α, macrophage inflammatory protein 1-alpha; MMPs, matrix metalloproteases; OC-STAMP, osteoclast stimulatory transmembrane protein; RANKL, receptor activator of nuclear factor kappa-Β ligand; SDF-1, stromal cell-derived factor 1; TGF-β, transforming growth factor beta; TNFα, tumor necrosis factor alpha; TRAP, tartrate-resistant acid phosphatase; VEGF, vascular endothelial growth factor.*

#### *Medical Therapy of Giant Cell Tumor of Bone DOI: http://dx.doi.org/10.5772/intechopen.97114*

surface markers expression [22, 37]. Additionally, mutations in cyclin D1, p53, and MET have been linked to malignant transformation and GCTB recurrence [22]. Biologically, Wnt/β-catenin and transforming growth factor beta (TGF-β) signaling pathways mediate the exacerbated proliferation of stromal cells in GCTB. βcatenin, cyclin D1, and p21 have been shown to be overexpressed in the nuclei of GCTB stromal cells [39]. Additionally, one study showed that protease activated receptor-1 (PAR-1) is also upregulated in GCTB downstream of TGF-β, via Smad3 and Smad4 [40]. In the study, PAR-1 knockout in GCTB stromal cells inhibited tumor growth, angiogenesis, and osteoclastogenesis in vitro and PAR-1 inhibition

GCTB physiopathology is not entirely understood, but there is compelling evidence that RANKL overexpression by mononuclear stromal cells plays a key role and elicits transformation of monocytic pre-osteoclast to osteoclast cells, ultimately resulting in osteolysis observed in these tumors [22, 41–43] (**Figure 2**). Accordingly, preclinical models have shown that OPG, a soluble decoy receptor for RANKL, inhibits monocyte activation and osteoclast differentiation [44].

*Simplified scheme of GCTB physiopathology. BMP-2, bone morphogenetic protein 2; CCR2, C-C chemokine*

*receptor type 2; CXCR4, C-X-C chemokine receptor type 4; DC-STAMP, dendritic cell-specific transmembrane protein; IL-1, interleukin 1; MCP-1, monocyte chemoattractant protein-1; M-CSF, macrophage colony-stimulating factor; MIP-1α, macrophage inflammatory protein 1-alpha; MMPs, matrix metalloproteases; OC-STAMP, osteoclast stimulatory transmembrane protein; RANKL, receptor activator of nuclear factor kappa-Β ligand; SDF-1, stromal cell-derived factor 1; TGF-β, transforming growth factor beta; TNFα, tumor necrosis factor alpha; TRAP, tartrate-resistant acid phosphatase; VEGF, vascular endothelial*

suppressed tumor growth and giant cell formation *in vivo*.

*Recent Advances in Bone Tumours and Osteoarthritis*

**2.2 Physiopathology**

**Figure 2.**

*growth factor.*

**58**

In GCTB, stromal cell-derived monocyte chemoattractant protein-1 (MCP-1/ CCL2) recruits bone marrow-derived CCR2/CXCR4-expressing monocytic osteoclast precursors from peripheral blood [45, 46]. Other soluble factors within GCTB microenvironment are chemotactic for myelomonocytic cells, including stromal cell-derived factor 1 (SDF-1/CXCL12), macrophage inflammatory protein 1-alpha (MIP-1α/CCL3), and M-CSF1 [26, 47]. Osteoclast precursors localized at GCTB microenvironment differentiate into active, bone resorbing, osteoclasts.

Different pre-clinical studies have shown that GCTB stromal cells with circulating mononuclear cells co-culture induces differentiation of osteolytic giant cells [41–43]. For differentiation to occur, RANKL expression in stromal cells is regulated by CCAAT/enhancer-binding protein beta (C/EBPβ), found to be overexpressed in GCTB [48], and also by parathyroid hormone-related peptide (PTHrP) in an autocrine manner [49]. Next, RANKL-induced cell fusion is costimulated by M-CSF and IL-34 [26] and enhanced by specific transmembrane proteins overexpressed in GCTB [50] and coupling components, like insulin-like growth factors (IGF) I and II [51].

RANK pathway activation in giant cells leads to up-regulation of nuclear factor of activated T cells c1 (NFATc1), an auto-regulated key transcription factor responsible for regulating expression of important genes involved in bone resorption, like cathepsin K or β3-integrin [52]. Cathepsin K is involved in initial steps of bone resorption, degrading collagen type I and remodeling the bone matrix, allowing migration. As bone resorption starts, TGF-β entrapped in bone matrix is activated by matrix metalloproteases (MMPs), stimulating giant cell migration [46], which is mediated by αvβ3 integrin attachment to the bone matrix [53].

MMPs have an important role in GCTB physiopathology. Apart from the abovementioned role in giant cell migration via TGF-β activation, MMPs influence other major aspects within the tumor microenvironment, like angiogenesis, invasion, and metastatic development. MMP-2 and MMP-9 are key in all these processes [22]. In GCTB, the extracellular matrix metalloproteinase inducer (EMMPRIN) is responsible for inducing MMP expression. Higher EMMPRIN expression at multinuclear osteoclast-like giant cells has been observed in stage III GCTB, probably regulated by RANKL from stromal-like tumor cells [54].

As previously mentioned, metastases are extremely rare in GCTB and there are no clues on molecular or physiopathological events related with GCTB metastization to date.

#### **2.3 Tumor markers**

Pathophysiology of GCTB progression remains unclear and prognostic factors, treatment targets, and predictive biomarkers represent unmet needs.

Histologically, ambiguous giant cell-rich lesions including benign GCTB, chondroblastoma, aneurysmal bone cyst, central giant cell granuloma of the jaw, and malignant giant cell–rich osteosarcoma are often found, especially as small biopsy or curettage specimens [22]. In these cases, *H3F3A* gene p.G34W mutation can be used in the differential diagnosis, as it is almost GCTB-exclusive [30, 55]. Approximately 90% of GCTBs display the p.G34W mutation, with minor subsets (<2%) displaying p.G34L, p.G34M, p.G34R, or p.G34V variants. H3F3B p.K36M is the H3.3 mutation found in the vast majority (90–95%) of chondroblastomas [30].

H3.3 p.G34W mutant-specific immunohistochemistry (IHC; clone RM263, commercially available) is a highly sensitive and specific surrogate marker of H3F3A p.G34W mutation in GCTB [56–58], being useful for practical diagnosis in primary [58] or recurrent, metastatic, and secondary malignant GCTB [59]. Although denosumab therapy may decrease p.G34W expression [22], evidence

shows that spindle cells and cells in and around immature bone in denosumabtreated GCTBs are H3.3 p.G34W-positive by IHC, with H3F3A mutations consistently detected in corresponding samples [56, 58, 60, 61], which may predict relapse risk [55].

Finally, it has been suggested that high RANKL, IL-6, TNFα, SDF-1, and MCP-1 expression may help predict GCTB metastatic potential and prognosis, warranting

Treatment options for localized GCTB include *en bloc* resection or curettage with or without local adjuvants, like phenol, liquid nitrogen, or polymethylmethacrylate [70]. Radiation therapy (RT) can also be used as an alternative to surgery for local control, with 5-year local control rates of 80% [71]. However, RT is associated with risk of malignant transformation into high-grade sarcoma, making surgery the preferred option when possible. Contrarily to palliative care in irresectable or distant disease, systemic neoadjuvant or adjuvant therapy with the RANKL-binding fully human monoclonal antibody denosumab is still not established [70, 72].

Denosumab is a fully human monoclonal antibody (IgG2) that binds with high affinity and specificity to RANKL [74], thereby inhibiting osteoclast-mediated osteolysis. Given GCTB pathophysiology and its association to RANKL/RANK

In patients with resectable GCTB, adjuvant denosumab at a 120 mg dosage administered subcutaneously every 28 days, with additional loading doses on days 8 and 15 on the first month, during 6 months after complete resection has been approved by both the Food and Drug Administration and European Medicines Agency [72, 75, 76]. However, this treatment is still debated. Studies supporting its use in the adjuvant setting are scarce and mostly rely in level IV evidence. Conversely, evidence from a systematic review by Luengo-Alonso [72] favored adjuvant denosumab, which showed a positive histological and clinical (pain relief) response. In patients with unresectable GCTB (either primary or recurrent) or when complete excision is possible but post-surgical severe morbidity and functional impairment is expected, neoadjuvant denosumab should be started (same dosing scheme as above) and response to treatment evaluated. Should the patient respond to denosumab and surgery be feasible with acceptable morbidity, then complete excision and possibly adjuvant denosumab for six months should be considered. On the other hand, the optimal denosumab duration is still debatable when treatment response is suboptimal or in cases of sacral or spinal GCTB, multiple lesions (including pulmonary metastases), or patient's clinical ineligibility for surgery. Denosumab should be considered until progression or unacceptable toxicity (e.g., osteonecrosis of the jaw), provided at least partial response is achieved.

Bisphosphonates inhibit osteoclast-mediated bone resorption and are used in cancer patients, especially in bone metastases setting. In GCTB patients, denosumab is the preferred systemic treatment option. However, evidence regarding the use of adjuvant denosumab is not consistent and some studies show lack of benefit in local recurrence rates [77, 78]. Bisphosphonates, like zoledronic acid (ZA), can be an

further studies [69].

**3. Treatment overview**

*Medical Therapy of Giant Cell Tumor of Bone DOI: http://dx.doi.org/10.5772/intechopen.97114*

**4. Medical therapy**

**4.2 Bisphosphonates**

**61**

**4.1 Denosumab**

A treatment algorithm is depicted in **Figure 3**.

pathway, denosumab has proven effective in this disease.

Although rare, malignant GCTB may occur, and studies suggest that p.G34W mutation is preserved [55]. One report, however, showed loss of one *H3F3A* allele (probably the mutant allele) in GCTB malignant component, leading to negative p. G34W IHC [62].

p63, a member of the p53 family of transcription factors, has also been studied as biomarker in GCTB diagnosis. p63 immunostaining has been used in diagnosis of head and neck squamous cell carcinoma, prostate adenocarcinoma (negative for p63 in opposition to p63-positive benign prostatic tissue) [63], and poorly differentiated squamous cell carcinoma [64]. p63 has also been shown to be highly expressed in GCTB mononuclear neoplastic cells [65–67], but its usefulness is still to be determined. A meta-analysis of eight different studies including 335 GCTB patients showed that p63 is a helpful marker for GCTB diagnosis in critically ill patients, although it cannot be recommended as a single definitive diagnostic marker [68].

#### **Figure 3.**

*Flowchart of GCTB treatment. Adapted from NCCN guidelines – Bone cancer [73]. CT, computorized tomography; MRI, magnetic resonance imaging; RT, radiation therapy; SC, subcutaneous.*

Finally, it has been suggested that high RANKL, IL-6, TNFα, SDF-1, and MCP-1 expression may help predict GCTB metastatic potential and prognosis, warranting further studies [69].
