Introduction

Although it is a relatively rare malignancy, adrenocortical cancer is a devastating diagnosis for those patients who are afflicted and for their families. Resectability is the prime determinant of prognosis. For those with disseminated disease, chemotherapy options are few and lack sufficient efficacy. Encouraging progress is starting to be made with the recent conclusion of accrual of both the FIRM-ACT and GALACCTIC trials focused on ACC. Both trials accrued ahead of schedule demonstrating the support the ACC community of researchers, clinicians, and patients toward finding new more effective treatments. In this review, we will review the current status of emerging potential treatments for ACC.

Targeting the IGF Signaling System

Molecular analyses of human cancers including adrenocortical cancer have identified perturbations in pathways regulating cell growth and survival. Current efforts to target the insulin-like growth factor type 2 (IGF2) receptor as a potential therapeutic strategy in ACC is the direct result of analysis by several groups of investigators that have shown consistently high expression of IGF2 in ACC. IGF2 effects are mediated through its receptor IGF1R resulting in activation of the PI3K/AKT/mTOR cascade and the RAS-MAPK pathway [1]. Several targeting strategies to block activation of IGF1R are being investigated. Monoclonal antibodies have been developed to the ligand or the receptor, such as cixutumumab (IMC-A12). At present, there is an ongoing randomized phase II trial comparing mitotane to mitotane and cixutumumab in patients with adrenocortical cancer. A small molecular inhibitor of IGF1R, OSI-906, is also undergoing investigation in ACC. A phase III trial is ongoing comparing OSI-906 to best supportive care in patients with ACC that has progressed despite standard chemotherapy.

Targeting the IGF1R pathway is a rational strategy in adrenocortical cancer. In 2009, Barlaskar et al. demonstrated by expression array analysis that IGF2 was overexpressed in ACC compared to normal adrenal cells and benign adenomas [2]. They also showed that the combination of mitotane and IMC-A12 was synergistic in retarding the growth of murine xenografts of the H295 adrenal cortical cancer cell line. Features of the IMC-A12 monoclonal antibody relevant to its potential as a cancer therapeutic are that it is specific for the IGF1R receptor, it may induce antibody-dependent cellular cytotoxicity, it is administered intravenously, and while it has a relatively long half-life, it does not cross the blood–brain barrier [3, 4].

There is concern that anti-IGF1R monoclonal antibodies may induce compensatory activation of the epidermal growth factor and insulin receptors [5]. In studies of the small molecule IGF1R inhibitor OSI-906, inhibition of the IGF1R and the closely related insulin receptor (IR) resulted in enhanced inhibition of the AKT pathway, as demonstrated by reduction in phosphorylated AKT [5]. Class-specific features of the tyrosine kinase inhibitors are that because there is 95% homology in ATP binding sites of the IR and IGF1R, there may be an effect on glucose metabolism. Hyperglycemia has been observed in the clinical trials involving OSI-906. This agent is orally administered and may cross the blood–brain barrier [6]. At the Translational Genomics Research Institute (TGen), in vitro studies using the ACC cell lines, H295R and SW13, showed that OSI-906 is cytotoxic with IC50 levels of 0.59 and 32.5 μM, respectively [7]. In H295R murine xenografts, OSI-906 administered at 50 mg/kg showed 95% tumor growth inhibition at 28 days (personal communication, OSI Oncology). In the phase I studies of OSI-906, 12 patients with ACC were enrolled. One of these patients had a partial response and four had stable disease by RECIST. The patient who exhibited a partial response is a 35-year-old woman with metastatic ACC to her lungs. She had been treated previously with mitotane, etoposide, doxorubicin, and cisplatin (mitotane-EDP) to achieve a partial response after 4 cycles. By 6 cycles, her tumor progressed. After 32 weeks, her primary right adrenal cancer and her lung metastases had decreased in size by 68%, using RECIST. Currently, the GALACCTIC phase III trial has completed its accrual goal of 135 patients. In this study, OSI-906 is being compared to best supportive care alone (i.e., placebo treatment) in a 2:1 randomization strategy to assess the potential efficacy of OSI-906 in patients whose ACC tumors have progressed on standard chemotherapy with mitotane or mitotane-EDP. The primary endpoint of this study is overall survival. The accrual for the GALACCTIC study was less than 2 years, a rapid interval given the rarity of adrenocortical cancer, attesting to the dedication of the investigators, advocacy groups, and patients to make progress against this disease. The results of this trial are eagerly awaited.

Beyond the IGF1R

As outlined above, there is a wealth of information indicating that there is a strong molecular rationale for targeting the IGF1R as therapy for ACC. However, it is worth bearing in mind that treatment targeting other tyrosine kinases may also provide significant benefit. Like most cancers, ACC is a complex disease that results from multiple genetic hits affecting many signaling pathways. Thus, it is worth examining other known RTK systems in ACC with a goal to determine which may also be potential therapeutic targets, both from the viewpoint of being important causative agents in the cancer as well as being potentially druggable targets (Table 1).

Table 1 Clinical trials of TKIs potential relevant for adrenocortical cancer
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EGFR

The epithelial growth factor receptor (EGFR, also known as ERBB1) is a tyrosine kinase whose extracellular domain binds and transduces the signal from EGF. There are three additional closely related family members, known as ERBB2, ERBB3, and ERBB4. EGFR family members are typical RTKs and comprise of extra- and intracellular portions linked by a single transmembrane domain. Signaling from this family of proteins occurs through ligand binding and the formation of homo- or heterodimers among family members. Although EGFR has ligand binding, dimerization, and tyrosine kinase domains, the same is not true for each of the family members. For example, the ERBB2 protein exists in an “open” configuration which does not require ligand binding. Also, ERBB3 lacks the kinase domain, so it can only transduce signals as a heterodimer [8]. The EGFR is expressed in >90% of tumors, although levels are similar to those observed in normal adrenal tissue [9, 10]. Surprisingly, EGF exhibits very low levels of expression in most tumors; however, the EGFR family can bind to other growth factors such as TGFα, which is expressed at high levels in ACC [10]. Because of these observations, a clinical trial was undertaken to assess the effectiveness of the EGFR TKI gefinitib as a single agent in ACC. This trial of 19 ACC patients did not show significant response [11]. Additionally, a small trial of another EGFR TKI erlotinib in combination with the cytotoxic agent gemcitabine was carried out as salvage therapy in advanced ACC [12]. Unfortunately, this combination of therapies produced minimal (if any) benefit and the trial was not expanded. No further trials have yet been undertaken. In retrospect, the poor success rate of gefinitib as a single agent is not surprising. First of all, this agent is very specific for the EGFR, such that signals transduced by other members of the receptor family or by receptor heterodimerization are not blocked. There are some data to suggest that ERB2–ERB3 signaling pairs are the most potent growth stimulators in other types of cancer, so that Iressa would not have worked. Newer agents that may inhibit receptor dimerization (such as the mAb pertuzumab) may be more effective [8]. As discussed below, EGFR family members may be valuable tools in combination therapy for ACC.

FGFR

Aside from the IGF1R, members of the fibroblast growth factor receptor (FGFR) family appear to be among the most commonly overexpressed RTKs in ACC [13–16]. There are four members of the FGFR gene family, FGFR1–4 [17], and elevations in FGFR1 and/or FGFR4 have been observed in multiple studies. In one of these studies, upregulation of FGFR1 was shown to correlate with the malignant potential of the adrenal tumors [15]. Although the FGFR would seem to be a reasonable therapeutic target for ACC, clinical trials aimed at this pathway have not been undertaken. Therapeutic interest in the pathway has been stimulated by the observation that FGF signaling may also be activated by genetic translocations in multiple myeloma [18]. Part of the limitation on targeting FGF signaling has been the lack of agents targeted to this pathway. The small molecule dovitinib (CHIR-258/TKI-25) was developed as an FGFR inhibitor. This agent, which has activity against multiple receptors including VEGFRs, PDGFR, and others, has shown preclinical activity in vitro and in vivo in cells expressing a variety of FGFR isoforms [32]. The most advanced of these agents is known as CWP232291 and is currently being developed by the Choongwae Pharma Corp. in South Korea. This agent has shown activity against multiple myeloma cells in preclinical models both in vitro and in vivo by promoting degradation of β-catenin [33]. Although there is a lot of potential for antigrowth effects of agents targeted to this pathway, the widespread use of Wnt signaling for many cellular processes means that this research must proceed with caution due to concerns about significant toxicity [34].

Rationale for Combination Chemotherapy Strategies

To date, most clinical trials specifically targeted to ACC have focused on the use of single TKIs (Table 1). Although this information is valuable in establishing a baseline for therapeutic efficacy, this type of treatment seems destined to provide only modest benefit to patients. A more fruitful approach may be the use of multiple agents, a practice well understood not only from oncology but also in microbiology where multiagent therapy is the norm for difficult cases. This type of rationale is based on multiple factors. First, multiple signaling pathways typically are involved in cancer, such that (as discussed above) single-agent strategies may just shift the dominant signaling pathways to those that are not blocked. Second, resistance to single agents may occur by mutation of the target kinase, gene amplification, or upregulation of alternate pathways [35]. This latter effect could be blocked by using targeted therapies against these emerging changes. Third, strategies aimed at targeting multiple signaling pathways may benefit from therapeutic synergism and be significantly more effective than single agents used in sequential fashion. Finally, kinase inhibition may affect other factors contributing to tumor growth and spread, including those in the microenvironment.

In regard to synergy, there are well-documented interactions between the IGF1R and EGFR signaling pathways [5, 36, 37]. It has been demonstrated that tumors that are sensitive to IGF1R inhibitors rapidly upregulate signaling through the EGFR system as a means to resistance [38], and the converse is also true [37]. For this reason, combining agents targeting both of these pathways would seem a rational approach likely to enhance effectiveness of the agents. The interaction of the IGF and EGF signaling pathways has been demonstrated in multiple preclinical systems, and therapeutic cooperativity has been confirmed, regardless of whether antibodies or small molecule TKIs have been used [5, 36, 37, 39, 40]. Recently a clinical trial of the IGF1R-targeting OSI-906 and the EGFR-targeting erlotinib (originally OSI-774) is already underway. Phase I studies have shown that there is moderate but manageable toxicity of the combination therapy, and an expansion cohort using nonsmall cell lung cancer (NSCLC) patients was planned along with an upcoming phase II effort. This study indicates the feasibility of combination therapy and should lead to new and exciting trials in this area. Other combinations that may be worthy of consideration would be joint IGF1R/FGFR blockade.

Using TKIs in Novel Treatment Paradigms

Antiangiogenic Agents

Since the groundbreaking work of Folkman [41], there has been recognition that tumors require the development of new vessels and that this requirement presents a therapeutic opportunity [42]. Angiogenesis is driven primarily by signaling through the vascular epithelial growth factor (VEGF) pathway, and there are three VEGF receptors, VEGFR1 (Flt1), VEGFR2 (Flk1/KDR), and VEGFR3 (Flt4). VEGFR2 is thought to be the major mediator of blood vessel growth, although the others may also play roles. There is good in vivo evidence in model systems that targeting the vasculature is effective, including the observation that pretreatment with anti-VE-cadherin-linked agents to “normalize” the tumor vascular enhances subsequent delivery of cytotoxic chemotherapy [43]. The first antiangiogenic therapy to be approved was the monoclonal antibody bevacizumab, which targets the VEGFR. Initial studies demonstrated modest effectiveness against colon cancer [44], and it was subsequently approved for treatment of NSCLC and breast cancer as well [45]. Recent data have shed some doubt on the usefulness of the agent for breast cancer, and a reevaluation of this therapy is currently underway [46]. More recent agents that target the VEGFRs include the multikinase inhibitors sorafenib and sunitinib, which target these pathways among others. A trial of sunitinib as a single agent for ACC was not successful, although the trial was confounded by the fact that adequate serum levels of the drug were rarely obtained [47]. A second signaling system that seems to be important for angiogeneisis involves MET (hepatocyte growth factor) and its receptor HGFR. MET is typically expressed at high levels in the tumor microenvironment, and this signaling system appears to be an important secondary pathway for angiogenesis. A switch to MET-HGFR may be an important mechanism that accounts for resistance of tumors to therapies targeted to the VEGFRs, such that combination therapy has been shown to be effective in preclinical models [48]. Intriguingly, XL-184, a multikinase inhibitor currently in clinical trials, targets both VEGFR and HGF, making it an agent with potential promise in this area.

Antimetastasis Agents

Because metastatic disease is a common cause of morbidity and mortality in ACC, the emerging field of metastasis suppression may be highly relevant to the therapy of ACC tumors. Although a thorough review is the beyond the scope of this paper, many excellent reviews on this topic have been published [49–52]. In terms of therapeutic interventions, an intriguing target is stromal-derived factor 1 (SDF1, also known as CXCL12) and its receptor CXCR4. SDF1 is a secreted cytokine whose levels are increased by mutant forms of p53 [53] such as may be observed in ACC tumors [54]. SDF1–CXCR4 interactions seem to be important for homing of tumor cells to appropriate metastatic niches. This same process is involved in immune cell homing. The agent AMD3100 is a clinical agent in use that works through this mechanism. By blocking SDF1–CXCR4 interactions, it causes enhanced mobilization of bone marrow cells, which can then be used for bone marrow transplantation. Whether this could be used to block metastatic seeding is an avenue that may be worth pursuing.

Individualized Treatments

In addition to exploring the novel agents described above, investigators and patients are now considering future areas of study. One area of exciting research proposes to identify individualized treatment strategies which will allow selection of agent or combinations of agents or dosing schedules that will be the most effective for each patient. In order to better select treatment strategies for each particular patient, one must develop better markers of either drug sensitivity or resistance based on tumor biology. For example, investigators including the group in Wurzburg have shown that a marker of drug resistance, ERCC1, can predict a likelihood of failure of treatment with platinum agents in ACC [55]. Standard chemotherapy for ACC is currently the regimen of etoposide, doxorubicin, and cisplatin plus mitotane as proposed by the Italian group [56]. This regimen, in their phase II trial, was associated with a 49% response rate. It is, however, a fairly rigorous regimen associated with significant toxicity. The questions regarding this regimen remaining include whether the regimens dosing schedule and component agents should be altered based on the patient’s individual tumor characteristics. As demonstrated by the Wurzburg group, if tumor ERCC1 levels are high, then one is unlikely to have a response to platinum agents, so for these patients should cisplatin be omitted? Although this hypothesis has not been tested, the current literature at least suggests one should definitely include cisplatinum in the chemotherapy regimen for patients with ACC tumors expressing low levels of ERCC1. Our expression profiling of patient tumors shows that in ACC, there is overexpression of the TOP2A and low expression of the multidrug resistance gene MDR1 which should predict a favorable tumor response to treatment with doxorubicin. Further analysis, however, shows that there are three subsets with distinct tumor profiles regarding expression of the TOP2A and MDR1 genes. In one subset, TOP2A is low, so it seems unlikely that treatment with the drug targeting this gene would be effective. In another subset with increased TOP2A expression, where one might predict that doxorubicin may be highly effective, if MDR1 expression is also increased, one might predict then that the beneficial effect of the agent would be mitigated. Ideally, one would select a patient with high TOP2A and low MDR1 levels in their ACC tumor samples to treat with doxorubicin. Although this strategy holds promise, verification using in vitro drug testing or studying tumor response in vivo using mouse xenografts will likely be an important bridge between molecular analysis and therapeutic selection in individual patients.

Recent work in the Demeure and Bussey lab has also highlighted preclinical evidence for nanoparticle-bound paclitaxel (nab-paclitaxel) to be investigated clinically in ACC [57]. The therapeutic target for nab-paclitaxel secreted protein acidic rich in cysteine was overexpressed 1.5-fold by microarray in ACC (Bussey and Demeure, unpublished data). Nab-paclitaxel is approved in the USA for the treatment of breast cancer. Knowledge mining of our expression array data set showed strong concordance with published breast cancer expression data sets. Previous cell line work had suggested in vitro efficacy in the H295 ACC cell line [58] prompting efforts to pursue this lead. In these studies, efficacy was observed in vitro and in vivo with murine xenografts using either the H295R or SW13 ACC cell lines (Bussey and Demeure, unpublished data).

In addition to expression array profiling, one can examine tumors by comparative genomic hybridization (CGH) to identify genomic aberrations that may give insights into potential treatments. In our profiling of ACC tumors, we found that some genomic aberrations could be correlated to a relatively poor prognosis [59]. In some cases, the detection of focal genomic aberrations could expose indication of a therapeutic vulnerability or suggest resistance to agents. For example, in one ACC, our group found a focal amplicon in BRIP1 (BRCA1 interacting protein C-terminal helicase), a gene which in some cases be a target of germline cancer-inducing mutations (Fig. 1). BRIP1 is a part of the Fanconi anemia pathway involved in the repair of DNA double-stranded breaks by homologous recombination. BRIP1 may be a target of germline cancer-inducing mutations; one hypothesis is that amplification of this gene provides a selective advantage to this tumor. Increased expression of FANCJ (BRIP1) has been detected in invasive high-grade breast cancers and is associated with a relatively poor prognosis [60]. Inactivation of BRIP1 through LOH and mutation with the resulting loss of BRCA1–BRIP1 interaction results in impaired DNA repair, checkpoint control, and decreased tolerance to DNA damage. Consequently, one could surmise that this amplicon may signal that this patient’s tumor is resistant to DNA damaging agents such as cisplatin, mitomycin C, or a PARP inhibitor.

Fig. 1
figure 1

Detection of a focal amplification of the gene BRIP1 in an ACC by CGH following flow-sorting of nuclei based on DNA content. The bottom panel is a view of entire genome by CGH. The middle panel focuses on chromosome 17 shows small focal amplifications, with a vertical line showing the amplicon detailed in the upper panels. The top panel depicts a further magnification view of the amplification in chromosome 17q between the loci q22.2 and q23.2 demonstrating a very focal aberration, as called by the Agilent detection software. This aberration contains only the BRIP1 gene, as shown by the bar below the probes

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Perhaps in the not very far away future is the ultimate form of personalized cancer treatment based on whole genome sequence (WGS) analysis. The cost of NextGen sequencing has been drop** rapidly, and the analytic tools required to allow routine clinical use do not seem to be far away. Once can foresee a time where one can envision a scenario in which a patient has a biopsy of their ACC done for WGS. A team of consisting of oncologists, bioinformaticians, systems biologists, and pharmacologists review that data and design a treatment regimen targeting the genomic context of the individual patient’s cancer. In this way information becomes the key therapeutic asset, in a way never before seen.

Summary and Future Directions

The wealth of molecular data available about the pathogenic changes that occur in ACC has enabled the development of strategies that may be more effective for treating these tumors. There is currently excitement in the field regarding the use of IGF1R inhibitors, and the results of the GALACCTIC trial (OSI-906 IGF1R TKI) and other similar trials should be available shortly. While the ACC community eagerly awaits the final conclusions of the FIRM-ACT and GALACCTIC clinical studies, all are cognizant that ACC has not been cured. While new agents including IGF1R inhibitors are on the horizon, it seems unlikely that single-agent regimens will provide the most effective treatment for an aggressive tumor like ACC. There is rationale for the involvement of other signaling pathways, including the EGF and FGF families of ligands and receptors that may have direct antitumor effects. There is also likely a role of other therapies targeting other process of the cancer pathway, including angiogenesis and other aspects of the microenvironment. As these agents come online for clinical use, it will be important to remain vigilant regarding side effects, as patients with hormone-secreting tumors may be at increased risk for complications [61, 62]. For example, VEGFR-targeted agents caused hypertension and decreased wound healing, adverse effects that may be augmented in patients with cortisol-secreting ACCs. Novel genomic technologies offer the promise of individually targeted treatments. A greater understanding of the molecular oncogenesis afforded by the application of genomic technologies including whole genome sequencing should speed progress toward more effective treatments. To make progress in the field, the involvement of large multinational consortia is needed to evaluate combinations of agents or therapeutic strategies in clinical trials. It is particularly encouraging that the ACC community of patients, physicians, and advocates has demonstrated that clinical trials in ACC can be accrued successfully and ahead of schedule.