Toward a drug development path that targets metastatic progression in osteosarcoma.

Introduction
As is the case for many solid tumors, the problem of metastasis is the most
important cause of morbidity and mortality in osteosarcoma patients. Based on
historical data, over 80% of patients will progress to develop metastasis following
resection of the primary tumor alone, and even with the addition of chemotherapy to
primary tumor resection, approximately one-third of patients presenting with
localized disease will subsequently develop pulmonary metastases (1, 2).
Long-term outcomes for osteosarcoma patients with either localized or metastatic
disease have not substantively improved in over 30 years, however progress in our
understanding of metastasis biology now offers hope to address this unmet clinical
need. Recent studies have defined the existence of druggable targets linked to
metastatic progression of cancer (3-7). Many of these targets and associated
processes appear to specifically influence the progression of metastatic cells from
microscopic disease to that of grossly detectable lesions (8). The modulation of these targets using either genetic or
pharmacological approaches may have no measurable effect on established and grossly
detectable lesions at either the primary or metastatic locations (9, 10).
As such these agents are predicted to fail in conventional early phase human trials
that require regression of established disease (8, 11). Preclinical therapeutic
studies in a variety of cancer histologies now support this prediction; novel
therapeutic agents designed from an understanding of the unique vulnerabilities and
targets linked to metastatic progression are indeed active against metastatic
progression but may have no activity in the setting of measurable disease (12-14).
In order for novel agents that target metastatic progression to advance, clinical
trials conducted in the adjuvant setting, in the absence of measurable disease, will
be required early in the drug development path. As noted above, our past reliance
and requirement for regression of measurable lesions to advance therapeutic agents
in drug development for osteosarcoma has not been rewarding. Accordingly, rigorous
preclinical data will be necessary to support the evaluation of a drug whose
activity and therapeutic benefit may be limited to preventing progression of
existent microscopic disease, without the expectation of measurable anticancer
activity in conventional response-based clinical trials. To advance the development
of such novel therapeutics, a meeting of key opinion leaders and experts in the
fields of bone sarcoma biology, metastasis, preclinical cancer drug development
(including cancer biologists and veterinary oncologists), and the clinical
management of osteosarcoma patients (pediatric oncologists, medical oncologists,
radiation oncologists, and surgeons) was convened in Bethesda Maryland on April
6th, 2013, with the support of the QuadW Foundation, the
Children’s Oncology Group, and CureSearch. The goal of this meeting was to
establish a consensus “Perspective” on osteosarcoma drug development,
which would focus on the problem of metastasis and establish a consistent
translational path that could support the early evaluation of potential therapeutic
agents that uniquely target the metastatic phenotype.

Osteosarcoma Drug Development Infrastructure
With the overriding goal of improving long-term outcomes for patients, the
osteosarcoma community has initiated or participated in programs that can now
support the development and integration of novel agents into osteosarcoma therapy.
First, through the efforts of the QuadW - Children’s Oncology Group Childhood
Sarcoma Biostatistics and Annotation Office (CSBAO), a robust and clinically
annotated osteosarcoma biospecimen repository is now available to fuel biological
investigations (J. Glover Personal Communication; ASCO 2013 Abstract). This
repository has been linked to a relational database that will be progressively
annotated with biological analyses performed on these tissues thereby allowing for
the first time in silico analysis in this disease. Second, the NCI
TARGET (Therapeutically Applicable Research to Generate Effective Treatments)
initiative is expected to deliver a greater understanding of osteosarcoma genomic
targets that may be matched with existing or novel cancer therapeutics (http://ocg.cancer.gov/programs/target). Third, the Pediatric
Preclinical Testing Program (PPTP) has an established infrastructure to test novel
agents for their activity against primary tumor growth in a diverse set of
osteosarcoma xenograft models (http://pptp.nchresearch.org/) (15). Finally, the NCI Comparative Oncology Program has established a
preclinical and translational consortium (COTC) that can rapidly evaluate the
therapeutic value of novel agents in pet dogs that have naturally developed cancers
most notably including osteosarcoma (https://ccrod.cancer.gov) (16).
With progress on these fronts, it is now feasible to integrate advances in our
understanding of osteosarcoma and metastasis biology with preclinical and
translational studies as a means to prioritize agents for evaluation in patients. It
is recognized that in order for the successful implementation of this integrative
approach, several existing drug development approaches, perspectives and resources
will need to be re-focused on the problem of metastatic progression rather than
regression of measurable cancer lesions alone.

The Quagmire for Osteosarcoma Metastasis Drug Development
The process of metastasis in osteosarcoma patients appears similar to
patients with other solid tumors. The steps associated with the metastatic spread of
cancer cells from a primary tumor to a distant secondary site involve a complex set
of discrete processes that are in many ways distinct from those associated with
primary tumor growth (17-23). Most metastasis biology studies suggest
that cancer cells readily gain entry to the circulation from the primary tumor and
that the majority of circulating cancer cells successfully arrive and extravasate at
the distant secondary site; however, only a small minority of cells are able to
survive at the distant and foreign microenvironment. Indeed, managing this critical
stage of vulnerability is a defining feature of metastatic cells (24). Through a combination of selective and
acquired events involving both genetic and epigenetic processes, metastatic cells
are distinguished from non-metastatic cells and are able to accommodate and adapt to
the stresses incurred during metastatic progression (25). In some cases the same oncogenic events linked to primary tumor
formation and maintenance are also responsible for facets of the metastatic cascade;
whereas other events are likely more intrinsically linked to the unique features of
metastatic biology provided by metastasis specific genes and gene regulation (26, 27).
As such there are unique targets and processes (often druggable) that may drive the
progression of existent microscopic metastatic cells to grossly detectable
lesions.
There are now sufficient experimental data to believe that the progression of
single metastatic cells to established lesions occurs after patients present with
apparently localized disease and continues after the development of radiographically
detectable lesions. First, it is likely that those cells that are able to complete
the steps of the metastatic cascade will subsequently metastasize to other parts of
the same secondary organ or to distinct secondary sites late in clinical
presentation (20, 28, 29). Second, it is
reasonable that tumor cells remain dormant as quiescent single cells for long
periods of time before they establish colonies of micrometastases in which a balance
of proliferation and apoptosis exists, and before they progress to detectable
lesions (19, 30, 31). Finally, it remains
unclear if this period of metastatic dormancy occurs at the secondary site
(i.e., in the lung in the case of osteosarcoma) or in a
so-called sanctuary sites (i.e., the bone marrow) with subsequent
and therefore late spread to the eventual clinical secondary site (32-35).
Accordingly, it is reasonable that targeting metastatic progression, particularly at
the secondary site will provide clinical benefit to patients in all stages of
presentation (i.e., it is not too late to target the metastatic
cascade even after a patient develops metastasis).
Recognizing the imperative to assess new therapeutic agents that target the
metastatic phenotype, a consensus on the nature of preclinical data needed to
advance the clinical development of an anti-metastatic agent is necessary. As this
necessary translation is planned, it is important to recognize that decisions to
advance a therapeutic agent to clinical development in the adjuvant-setting may need
to be made without any prior evidence of anti-tumor activity in human patients. As
outlined above, using input from experts in the field we now propose a consensus
“Perspective” towards this preclinical to clinical translational drug
development challenge (Table 1). An important
outcome of having a consensus on the types of data that are determined to be
valuable, as a novel agent is proposed for translation, is that preclinical
investigators will have a clear sense of what may be expected and similarly that
translational groups will be clear on what they may expect as they evaluate and
review therapeutic agents for potential clinical development. In addition to
providing a clear consensus on the types of data that may be useful for translation
of agents that target metastasis, Table 1
also provides a mechanism to compare or prioritize agents based on these data.
Importantly, Table 1 is not intended to
prescribe “go” or “no go” decisions on the suitability
of potential agents, but rather serves to provide a consistent framework to
objectively value and ascribe quantifiable merit to a list of novel agents being
considered for translational assessment. In Table
1, vertical columns represent discrete translational data types that may
be available for consideration in the preclinical to clinical translation of a novel
therapy that targets metastasis. Within each column, a Progressive Merit Score (PM
Score) is assigned an integer value between 1 and 6, commensurate with the potential
“value” of the data in that category. Similarly across columns, a
Relative Merit Score (RM Score) across data types is assigned an integer value
between 1 and 3, and commensurate with our perceived “value” of that
data-type to this drug development question. Using the PM Score (within a Data Type)
and the RM Score (across Data Types), their product (PM Score × RM Score) is
used to generate a Cumulative Relative Dataset Merit Score which than can be
assigned and compared among distinct data-sets for a specific translational
therapeutic opportunity. The guidance provided in Table 1 will provide a collective understanding of the necessary and
optimal data set needed to advance therapeutic agents with activity against the
metastatic phenotype and in-so-doing, will help prioritize those agents for clinical
development in patients with osteosarcoma.
As outlined above the pattern of failure for osteosarcoma patients continues
to involve the predictable development of metastasis to the lungs despite effective
and complete control of the primary tumor. Despite attempts to intensify therapy,
there has been a failure to decrease the development of metastasis and improve
patient survival over the past 30 years. As such there are no recent
“historical controls” that can be used as positive
“controls” to assess the scoring system. Accordingly validation of the
proposed approach will require prospective studies of novel therapeutic agents that
are first evaluated through the proposed scoring system, that then move on to human
clinical trials. The recent endorsement of the details outlined in this manuscript
by the National Cancer Institute Pediatric and Adolescent Solid Tumor Steering
Committee (PASTSC) will serve as a starting point for future discussions which will
lead to the potential integration of the proposed scoring system for the
prioritization of novel agents planned for clinical evaluation in pediatric
osteosarcoma patients. Accordingly, there will be an opportunity over time to test,
validate and modify the scoring system prospectively. As a means to demonstrate the
feasibility and future use of the scoring system, Table 2 provides examples for how the scoring system can be applied, in
this case by using therapeutic agents that have been recently evaluated in
osteosarcoma patients. These agents include liposomal muramyl-tripeptide
phosphatidyl-ethanolamine (L-MTP-PE) and inhaled granulocyte-colony stimulating
factor (GM-CSF), (36-38). Based on supportive preclinical data and phase II trials
in osteosarcoma, L-MTP-PE was advanced to a phase III trial in osteosarcoma. The
study included a factorial design using event free survival (EFS). No improvement in
EFS was seen within this factorial design; however, a subsequent post-hoc analysis
revealed an 8% improvement in survival in patients (36). All results, including the post-hoc analysis of survival, were
interpreted to be supportive of substantial evidence of effectiveness by the
European Medicine Agency and led to the recent approval of this agent in Europe for
patients with osteosarcoma. With a focus on the primary study endpoint of EFS, the
US Food and Drug Administration did not interpret the data to be supportive of
substantial evidence for effectiveness and the drug was not approved. The score for
L-MTP-PE, using our described scoring system (Cumulative Relative Dataset Merit),
was 60. In the case of the second example, inhaled GM-CSF was advanced into a trial
of first lung relapse osteosarcoma patients based on evidence supportive of the
feasibility of inhaled cytokine therapy. In the first lung relapse population and
within the constraints of the executed trial there was no evidence immune modulation
or antimetastatic activity demonstrated in patients (37). The Cumulative Relative Dataset Merit score for GM-CSF was 26.
Based on the two examples presented above it is clear that a broad range in scores
will be derived from the proposed scoring method. Indeed, it is reasonable that
these broad scoring possibilities will allow the prioritization of novel agents and
allow the hypothesis suggested by the proposed scoring method to be testable over
time.

A Proposed Mechanism to Value and Prioritize Preclinical and Translational Drug
Development Data (Table 1)

Target biology and expression
The most valuable drug targets, as they relate to the problem of
metastatic progression are those with functions that are fundamentally linked to
the pathogenesis of micrometastatic progression. It is optimal for these targets
to be expressed in micrometastatic cells. Although there are initial targets
that have been identified with these credentials, additional studies are needed
to expand the list of potential target candidates. Tissues from metastatic
lesions and matched primary tumor tissues from the same patients are not widely
available at this time and would provide a broader understanding of target
expression profiles and their dynamics during metastatic progression. Expanding
existing biospecimen efforts to collect clinically annotated tissues throughout
the course of disease presentation and progression is required in order to
better understand the development of metastases in osteosarcoma.

Drug mechanism of action and pharmacodynamics
It is likely that a more detailed understanding of mechanism of action
(MOA), and associated pharmacodynamic markers of effective therapeutic exposure
and target modulation in tumor and surrogate tissues will be needed for agents
that target metastasis and metastatic progression compared to agents that may
act on measurable disease. Since it is not likely that toxicity will be a
primary determinant of dose selection with biologically defined therapeutics, an
understanding of MOA and pharmacodynamics may be critical in the definition of
drug dose and schedule. Furthermore, it is widely recognized that the complexity
of the metastatic cascade is difficult to model in vitro, as such the use of
multiple (distinct) in vitro or preferably ex vivo assays
(i.e., Pulmonary Metastasis Assay; PuMA; (39)) of metastasis should be considered for
defining early evidence of therapeutic activity and more importantly to
elucidate mechanisms of action for a metastasis-targeting therapeutic.

Preclinical and murine models
Data demonstrating the activity of a novel therapeutic agent, at
pharmacologically achievable exposures in several distinct murine cancer models
is considered important for the development of all cancer drugs. The use of
experimental metastasis models (tail vein injection) that result in the seeding
of lung with cancer cells are valuable to “screen” potential
therapeutics, however, the use of orthotopic models of osteosarcoma that include
surgical management of the primary tumor and spontaneous pulmonary metastasis
should be prioritized as a means to more fully demonstrate the value of a
therapeutic approach. Genetically engineered models of osteosarcoma have now
been described and may be used in drug evaluation (15, 40). Genetically
engineered and other syngeneic models will be essential for therapeutics that
modulate the immune response as part of their mechanisms of action. It is
understood that the magnitude of a therapeutic response will be part of the
basis to prioritize one therapeutic outcome against another. As such, it is
essential that the variables that influence the behavior of a model and
therefore the magnitude of potential responses are considered
(i.e., delivered cell number, background of the mouse
strain used, time of treatment initiation) when comparisons between studies (and
between therapeutic agents) are made.

Canine osteosarcoma
Beyond the well-recognized difficulties with drug development in
osteosarcoma an important opportunity has been delivered by nature through the
spontaneous development of osteosarcoma in pet dogs (41). The opportunities of this comparative approach to
cancer drug development have been reviewed elsewhere (42). Biological, histological and genomic features of
osteosarcoma in dogs and humans are highly similar and have provided a basis to
evaluate novel therapeutics in dogs with osteosarcoma (43, 44). As part of
the broader field of comparative oncology, translational drug development
studies in dogs with osteosarcoma have been used to define dose and schedule for
therapeutic agents through rigorous pharmacokinetic-pharmacodynamic endpoints
that can involve serial biopsies of tumor and collection of biological materials
(i.e., normal tissue surrogates) before and after exposure
to a novel therapeutic (16, 45). Modeling of such dose-finding studies
for agents that target metastasis may be an important use of the dog as a model.
However the greatest value of the dog with osteosarcoma as it relates to this
“Perspective” is the opportunity to conduct studies in the setting
of micrometastatic disease. In such studies dogs will undergo management of the
primary tumor and then in the adjuvant setting receive investigational agents
alone or in combination with conventional chemotherapy backbones that are
similar to those used in human patients. Through the integration of imaging
endpoints, metastasis-free interval or survival may then be used to evaluate and
compare different doses and schedules of investigational agents. Through the
availability of a multi-center consortium of veterinary centers led by the
National Cancer Institute (Comparative Oncology Trials Consortium - (https://ccrod.cancer.gov) and the high prevalence of
osteosarcoma in dogs, multiple studies (or study arms) may be successfully
accrued in a time period that would allow comparison and prioritization of
agents for evaluation in human patients. It is likely that observed activity in
the adjuvant setting in the dog model would provide the most compelling data for
the value of a novel therapeutic that may target metastatic progression.

Pharmacokinetics
The nature and type of pharmacokinetic data needed to advance an agent
that targets metastatic progression is not likely to be different from
conventional cancer therapeutics. In the preclinical setting, studies should be
conducted at exposures that are likely to be achieved in human patients. It is
reasonable that studies of distinct treatment regimens (dose-schedule) in
patients may be important to optimize therapeutic responses in the adjuvant
settings. It is also important that these exposures are safely maintained during
what may be extended treatment intervals (i.e., during the
period of minimal residual disease).

Human clinical data
For agents that target micrometastatic progression, early human clinical
trials will continue to focus on tolerability. As part of the safety assessment
of these agents their use in the setting of a conventional osteosarcoma backbone
(i.e., methotrexate, doxorubicin and cisplatin) will need
to be established. Furthermore, as noted above, it is reasonable that the
duration of assessment of tolerability will need to be extended given the
expectation that novel agents that target metastatic progression may require
prolonged treatment exposures. As described under pharmacodynamics, given the
likely absence of response in a measurable tumor, early phase human trials
should optimally include pharmacodynamic endpoints that will provide confidence
on the adequacy of exposure and of the potential effectiveness of the exposure
in accessible biospecimens. Unlike many other cancer histologies, clinical
trials that assess the activity of therapeutic agents against metastatic
progression have been successfully completed and are currently underway in
osteosarcoma patients. These trials including the evaluation of MTP-PE (46), GM-CSF (47), and a src tyrosine kinase (SARC012; http://sarctrials.org/Open-SARC-Trials) inhibitor were possible
given the unique pattern of metastatic progression in osteosarcoma patients that
includes the lung as a target organ and the fact that surgical resection of
metastases is considered to be part of the standard of care. As novel trial
designs are considered there is a need to prioritize longitudinal endpoints of
survival and metastasis-free interval and to ensure that accrual and completion
of studies can occur in a reasonable time based on careful consideration of
eligibility criteria and inclusion of multiple partners including both pediatric
and adult oncology (9).

Conclusion
Improvements in long-term outcomes for osteosarcoma patients require a drug
development path that prioritizes agents with activity against metastatic
progression and not necessarily regression of measurable lesions alone. This
approach may also improve outcome for patients with more common cancers too. The
successful development of such agents demands a rigorous preclinical data set, since
we may not rely on early human clinical trials of tumor regression to support the
development of these potentially valuable therapeutic agents. This
“Perspective” provides reasonable guidance to consider and prioritize
such preclinical data in osteosarcoma. The use of these guidelines will assist
investigators in conducting studies that are believed to be most valuable in the
assessment of agents that uniquely target metastatic progression. Similarly the use
of these guidelines will allow more consistent evaluation and comparison of
potentially active agents as they are considered for clinical translation. It is
reasonable that after sufficient experience is gained through the use of these
guidelines that improvements and refinements can be made so as to optimize the
preclinical and translational development of drugs in osteosarcoma.