Dear Editors,

Denosumab is a human monoclonal IgG2 antibody that binds selectively and with high affinity to the receptor activator of nuclear factor-κB ligand(RANK-L) and pharmacologically mimics the effect of osteoprotegerin on RANK-L [1, 2] thereby blocking the binding of RANK-L to the receptor activator of nuclear factor-κB(RANK) [1]. Denosumab rapidly decreases bone turnover markers resulting in a significant increase in bone mineral density and reduction in fracture risk [3].

Osteonecrosis of the jaws (ONJ) was initially erroneously ascribed to being a later chemotherapy effect [2, 4]. ONJ became one of the most discussed adverse events in advanced malignancy [5]. Importantly, ONJ has never been reported to be associated with other pharmaceutical agents, except bisphosphonates [2, 4].

The case of a cancer patient who developed ONJ without previous history of irradiation or bisphosphonate administration has been published [6]. In this patient, ONJ was reported to have healed completely following sequestration, 15 months after drug discontinuation [6]. Despite the fact that this is the only case published to date, additional evidence published in scientific meeting proceedings suggest that the condition of ONJ may not be solely associated to bisphosphonates [7, 8]. Results from two randomized clinical trials (RCTs; NCT00321464 [8], NCT00330759 [7]) of denosumab in cancer patients with bone metastases report that ONJ occurred infrequently [7, 8] (Table 1). Both studies have current or prior intravenous or oral bisphosphonate administration in their exclusion criteria [7, 8], thus bisphosphonates as an etiopathological factor for ONJ in those participants who received denosumab can be ruled out. Since ONJ has not been previously described to be associated with other drugs agents administered to cancer patients [2], it can be suggested that these cases of ONJ are related to denosumab administration.

Table 1 Osteonecrosis of the jaw (ONJ) adverse effect cases reported from preliminary results of clinical trials comparing head to head denosumab with zoledronate for the treatment of bone metastases in patients with cancer
Full size table

The role of dosing interval and cumulative dose appears to be important regarding development of denosumab-related ONJ. All denosumab RCTs published to date include a dosing interval longer than 3 months and a cumulative dose of not more than 210 mg per 6 months [3]. None of these studies does report any cases of ONJ. On the contrary, preliminary results of both RCTs studying denosumab for the treatment of bone metastases in cancer patients include a monthly dosing interval and a dose of 120 mg per month [7, 8]. It is evident that denosumab-related ONJ could be a dose-related adverse effect. This latter argument has also been reported to apply to bisphosphonate administration [4, 9]. ONJ has been reported to be a much more common event in those patients receiving bisphosphonates for the treatment and prevention of cancer-related skeletal events (mainly intravenously), rather than in those patients receiving bisphosphonates (mainly orally) for non-malignancy indications [10, 11]. Similar to bisphosphonate-related ONJ pharmacosurveillance and reporting history [4], one could anticipate that broad introduction of denosumab into clinical practice, would allow for the recognition of the denosumab-related ONJ adverse effect in a much wider spectrum of prescription indications, including those for non-malignancy [10].

ONJ has been reported merely from trials of denosumab in patients with metastatic cancer. Thus, one could argue that it is the presence of metastases in the jaws of those patients that has been reported to be denosumab-related ONJ. Similar arguments have been presented prior to the recognition of bisphosphonate-related ONJ [2, 4] resulting in delayed recognition of ONJ as an adverse effect of bisphosphonates [4]. Based on currently available limited evidence, it could be difficult to reject such arguments; however, the previous paradigm of a relatively uncoordinated safety effort for bisphosphonate-related ONJ reporting [4] dictates that it may be important to early inform clinicians about this possible new adverse effect of denosumab.

It has been hypothesized that the introduction of new anti-resorptive agents to the clinician’s quiver could allow for the elimination of ONJ adverse effect [2]. It appears that denosumab cannot be such an anti-resorptive agent; more likely the medical community is probably facing a new agent that can induce ONJ. This association appears to be somewhat dose-related and this is a resemblance with bisphosphonate-related ONJ. Common plausible mechanisms for the etiopathogenesis of both denosumab- and bisphosphonate-related ONJ would encompass defective osteoclast differentiation, function, survival, and “fatigue” [12]. When compared to bisphosphonates, denosumab exhibits the advantage of short clearance time. Thus, more feasible treatment and earlier healing of denosumab-related ONJ when compared to bisphosphonate-related ONJ could be anticipated.