Here, we attempt to better define opportunities for optimizing pazopanib as a candidate therapeutic in ATC. We had previously found pazopanib monotherapy to have disappointing clinical activity in ATC (11
); therefore, we investigated combinatorial therapeutic approaches that led to the discovery of enhanced antineoplastic effects when combining pazopanib and antimicrotubule inhibitors in ATC model systems. Enhanced combined effects were attributable to inhibition of aurora A by pazopanib, and inhibitors of aurora kinases (and perhaps inhibitors of cell cycle–critical kinases in general) may represent attractive candidate therapeutics in ATC, especially when combined with microtubule inhibition.
Pazopanib/paclitaxel synergy was observed not only in vitro but also in vivo: Pilot anecdotal data from an ATC patient treated with the combination suggested marked and durable regression of metastatic disease. On this basis, a Radiation Therapy Oncology Group (RTOG) randomized clinical trial has been developed and activated, comparing paclitaxel monotherapy to the pazopanib/paclitaxel doublet when administered in parallel with intensity-modulated neck radiation therapy in ATC (ClinicalTrials.gov
From the mechanistic standpoint, the observed synergy was found to be associated with heightened paclitaxel-induced mitotic catastrophe, with the underlying mechanism defined here being relate to the inhibition of aurora A kinase by pazopanib. These data indicate that previously unanticipated “off-target” effects of pazopanib on aurora A may affect its clinical application and use, especially when combined with antimicrotubule inhibitors.
Our investigations into the mechanisms underlying the synergy between pazopanib and antimicrotubule agents in ATC have heightened our awareness of the potential significance of aurora kinases as candidate therapeutic molecular targets in thyroid cancers, especially in ATC, where we found a high degree of aurora A overexpressed at the mRNA level (and correspondingly increased at the protein level). The plausibility of aurora A as a candidate ATC therapeutic molecular target is further supported by data indicating that the augmentation of antimicrotubule agent–induced cytotoxicity by pazopanib can be recapitulated either by shRNA knockdown of aurora A or by the specific aurora A inhibitor MLN8237. Consequently, opportunities for combining antimicrotubule inhibitors with aurora A inhibitors more specific than pazopanib also appear to hold translational promise in ATC. Indeed, other groups have previously reported single-agent effects of pharmacological inhibition of aurora kinases in ATC (19
). Pazopanib/paclitaxel synergy has also been described in nonthyroid cancer models (17
), albeit apparently not previously in ATC, indicating that the finding may be generalizable across multiple tumor types and have application beyond ATC.
The relative extents to which aurora A and B may be important in cancer pathogenesis in general—or in thyroid cancer pathogenesis in particular—remain in dispute in the literature. Although our data indicate a role of aurora A in ATC, other studies point instead to a potential role of aurora B. In particular, Sorrentino et al
. reported aurora B overexpression in ATC relative to DTC and additionally found in vivo antitumor effects resulting from antisense aurora B treatment (24
). However, the ARO cell line used in some experiments was recently found not to represent thyroid cancer (25
) and the comparison group was DTC and not normal thyroid tissue, thereby drawing into question the voracity of reported results. Much akin to our present report, however, Wiseman et al
. instead found that aurora A was overexpressed in 41% of examined ATC patient samples, but that aurora B was not (26
). Hence, although aurora A collectively seems of greater importance in ATC pathogenesis, the precise relative contributions to which aurora A and B may be important in thyroid and other cancers of differing histotypes remain to be better defined.
It is also unclear why pazopanib monotherapy might lead to a decrease in the fraction of ATC cells in the G2
-M phase of the cell cycle, as opposed to the expected increase in G2
-M fraction in response to therapy with a specific inhibitor of aurora A (20
). We hypothesize that the decreased G2
-M fraction consequent to pazopanib treatment likely reflects the combined effects of pazopanib on multiple competing targets affecting cell cycle progression.
Recently, Gizatullin et al
. reported that cancers lacking intact DNA damage response checkpoints attributable to dysfunctional p53 pathways are more susceptible to apoptosis induction when treated with the aurora kinase inhibitor VX-680 (27
), raising the question of whether ATC, which frequently has mutated p53,may be especially sensitive to aurora kinase inhibitors. In our experiments, however, pazopanib produced overall qualitatively similar in vitro results in p53 wild-type (KTC2) and mutant (KTC3) cell lines, leaving this important question in need of further investigation.
In summary, our studies indicate that pazopanib and antimicrotubule inhibitors including paclitaxel combine synergistically in ATC models observed in association with heightened induction of mitotic catastrophe apparently attributable to the inhibition of aurora A by pazopanib. Further, aurora A (but not aurora B or C) is up-regulated in ATC, and specific aurora A inhibitors (such as MLN8237) or aurora A knockdown similarly combines synergistically with antimicrotubule inhibition in ATC. Moreover, we have also shown that pazopanib/paclitaxel synergy is observed not only in vitro but also in vivo, with pilot translation of these data producing encouraging preliminary results in human ATC. Collectively, we believe that the presented data provide compelling rationale not only for further evaluation of the paclitaxel/pazopanib combination in ATC but also for further study of the specific roles and therapeutic relevance of inhibition of aurora and other cell cycle–critical kinases in ATC, especially when combined with inhibition of microtubule function.