NANOSCALE COMPLEXES - A Novel Nanotechnology-Based Platform to Optimize Combination Cancer Therapies: Rational Development & Improved Delivery Using CombiPlex®

INTRODUCTION

Combination treatments for cancer continue to be developed in a manner largely unchanged since the inception of this approach over 50 years ago, where agents with non-overlapping toxicities are typically escalated to the highest dose possible based on their clinical use as single agents. This is done with the expectation that maximum efficacy can be achieved at the maximum tolerated doses (MTD) of the individual drugs. Emerging evidence indicates that this approach fails to recognize the critical role that drug-drug interactions, enabled by coordinated tumor cell exposure to multiple drugs, can play in enhancing efficacy by promoting synergy and reducing antagonism. In fact, the therapeutically optimal exposure of many combined agents may not be equivalent to their maximum tolerated exposure. Also, optimal efficacy of many combinations requires temporal control of tumor drug exposure to ensure that the degree of target inhibition/interaction is coordinated for interrelating pathways and processes. Taking advantage of these relationships in vivo requires that drug ratios be controlled following administration by coordinating the pharmacokinetics of the combined agents so that optimal ratios are exposed to tumor cells while avoiding antagonistic ratios.

Nanoscale drug carriers, such as nanoparticles and liposomes, are well suited for this application because they can be designed to synchronize the release of drug combinations following injection such that synergistic drug ratios can be maintained and delivered to tumors. We refer to this approach of controlling drug combination exposure in vivo as CombiPlex^®, a technology in which drug development activities focused on specific combinations are prospectively integrated much earlier in the drug development process. The sections below describe how CombiPlex addresses the challenges facing the traditional development path of many contemporary drug combinations and provide clinical proof-of-principle evidence that this approach can yield marked improvements in efficacy and patient outcomes.

CHALLENGES FACING THE TRADITIONAL APPROACH FOR DEVELOPING DRUG COMBINATION REGIMENS

The early advent of front-line cancer treatment regimens were composed of multiple cytotoxic agents designed to indiscriminately overwhelm, disable, or disrupt multiple cellular processes that are more active in tumor cells compared to healthy cells. More recently, the development of molecularly targeted agents (MTAs) has expanded at a rapid pace, with high expectations for breakthrough treatments in a broad range of cancer types. Initially, this class was predicted to provide improvements in patient outcomes due to highly specific targeting of tumor cell-signaling processes and decreased non-selective systemic toxicity. While there have been promising signs of activity in previously difficult-to-treat tumor types (eg, B-Raf inhibitors used to treat melanoma), responses to single agent treatments have often been transient.¹ Cancer cells use multiple signaling pathways to both grow and resist therapies; consequently, it is not surprising that single MTAs often have modest or transient activity due to interpathway communication and feedback loops that compensate for loss or blockade of a targeted signaling pathway. These types of compensatory intracellular signaling networks create a situation in which many combinations require simultaneous target inhibition to be effective.² Due to MTA target specificity, combination therapy is of even greater importance because of the need to target multiple pathways in order to contend with the biological heterogeneity of human tumors. As a result, increasing efforts have focused on combinations of MTAs, and for certain indications, this has led to FDA approvals.^3,4

The traditional drug development path focuses the majority of early stage (preclinical and Phase I/II) activities on the identification and development of new individual therapeutic agents aimed at targets that are believed to be important drivers in cancer progression. The effectiveness of any drug therapy is largely dictated by its pharmacokinetic and pharmacodynamic (PK/PD) properties. Consequently, these attributes are extensively studied and optimized to produce the most efficacious single agent treatment regimen possible within the drug class and for a particular clinical indication. However, this diligence does not typically extend to the development of drug combinations in which large differences in PK/PD properties can lead to markedly different biodistribution and tumor exposure between two combined drugs. These differences are readily seen in the quantitative whole body autoradiography (QWBA) images of rats administered the combination of cytarabine plus daunorubicin (Figure 1). The differences in the in vivo behavior of the two drugs administered in their conventional forms are striking. Both drugs rapidly distribute throughout the entire body within 1 hour following administration that may present toxicity implications. After 24 hours, virtually no cytarabine remains within the body, yet daunorubicin persists with higher concentrations in several organs. Both the rate of elimination and the biodistribution profile of these two drugs are substantially different, such that the ratio of drug concentrations in various organs changes greatly over time.

Although drug combinations are widely used within oncology because they are typically more efficacious than each agent administered on its own, the current approach for developing combination therapies (Figure 2) is inefficient in terms of the ability to identify and optimize the dose and schedule of each agent within the combination and the time and patient resources needed to develop each agent to registration. While there may be in vitro and preclinical studies supporting the utility of two drugs in combination, each drug is developed individually until registration before combination studies are initiated. This usually results in quite different treatment schedules for each agent and consequently, a significant amount of effort and experimentation must be performed in patients when the agents are eventually combined for use in a Phase I combination trial. A multifaceted trial is typically required to determine the dose of each agent and the schedule of the combination, frequently conducted by clinicians employing an empirical approach. Often, the more active agent is used at high doses, while the less-active agent is substantially dose reduced. Only at that point can the proposed combination regimen be evaluated for efficacy in humans, and it is generally not known whether the optimal tumor exposure of the multiple agents is achieved due to differences in PK/PD properties of the individual agents. Drug interactions affecting efficacy are almost never considered or exploited. Furthermore, it is often unknown whether the degree of target inhibition associated with regimens for the combined drugs based on safety profiles will, in fact, be most efficacious.

Conventional combination dosing regimens are unable to coordinate combination drug exposure to tumor cells due to large differences in PK/PD properties of the individual drugs, and consequently, the approach commonly used is to saturate the body continuously with high doses of each agent in an attempt to achieve simultaneous and prolonged inhibition of the intended targets. This can lead to excessive and often dose-limiting toxicity due to the extensive exposure of healthy tissues to the drugs that compromises the utility of the combination and, if doses must be reduced, limits target inhibition. An example of this effect is the combination of MEK inhibitors with Akt inhibitors. When agents of these two classes were combined in a Phase I trial using repetitive and prolonged oral dosing schedules, significant GI and dermal toxicities were experienced that necessitated additional dose escalation/de-escalation schemes.⁵ While there was some evidence of anti-tumor activity, it was unclear whether these results reflected the optimal efficacy achievable due to the underlying tumor biology or if toxicities associated with uncoordinated PK and biodistribution precluded exposing the tumor cells to the optimal concentrations (ratios) of the two agents. Lack of certainty about drug delivery and maximal biological effect may cloud the interpretation of clinical outcomes.

THE COMBIPLEX ADVANTAGE: RATIONAL DEVELOPMENT OF COMBINATION PRODUCTS

Improving the effectiveness and efficiency of drug combination development requires changing the focus from developing single agents to registration prior to combining them in the clinical setting, to the rational development of drug combinations starting at the research and preclinical stage (Figure 3). CombiPlex offers the ability to optimize drug combinations for efficacy preclinically, rather than in the clinic, and in a manner that can be translated more predictably into the clinic. In vitro testing is used to identify drug-drug interactions and to establish the relationship between drug ratio and the degree of synergy/antagonism for simultaneous drug exposure in a broad range of tumor cells. Nanoscale drug carriers are then iteratively engineered to coordinate the PK/PD of the combined agents such that the optimal drug ratio for the combination identified in vitro is maintained over extended times following injection (Figure 3). Furthermore, nanoscale (eg, 20 to 100 nm diameter) particulate carriers, such as liposomes and nanoparticles, direct the distribution of drugs away from the majority of healthy tissues, while enabling preferential accumulation into sites of tumor growth due to increased vascular permeability of tumors. These features help overcome many of the problems associated with conventional combination regimens (in which the therapeutic window between efficacious and toxic doses is often very narrow) while at the same time also ensuring that optimal drug ratios are maintained with target cells exposed to sufficient concentrations of each drug to be maximally efficacious.

CombiPlex therefore minimizes the potential uncertainty of treatment outcomes due to the uncoordinated PK/PD properties of the combined drugs, as these properties are now dictated by the nanoscale carrier that maintains the drugs at the administered ratio. Drug ratio and tumor exposure are optimized at the preclinical stage, which allows starting clinical studies directly with the optimized drug combination product. Thus, CombiPlex (Figure 4) presents two major advantages over the traditional drug combination pathway (Figure 2). First, because CombiPlex introduces the combination in the clinic at the first-in-man stage, this avoids the redundant loop of Phase I/II testing of individual agents followed by re-evaluation in Phase I and Phase II trials as a combination. Second, CombiPlex nullifies the confounding influence associated with uncoordinated PK/PD of conventional combination regimens by ensuring that tumor cells are exposed to the combined agents at the optimal ratio. Therefore, CombiPlex-derived combination products will arrive at a more definitive Go/No-Go decision much earlier than the traditional model of combination drug development.

COMBIPLEX CASE STUDY: VYXEOS^TM (CPX-351)

Combination chemotherapy incorporating cytarabine plus an anthracycline, such as daunorubicin, has remained the standard-of-care for newly diagnosed AML for over 40 years despite many attempts to improve this treatment regimen by altering drug dose and schedule or incorporating new cytotoxic and molecularly targeted agents. This situation provided an opportunity to directly test the impact of the CombiPlex approach versus the traditional combination approach head-to-head in a clinical setting. VYXEOS is a CombiPlex-derived liposomal formulation of cytarabine and daunorubicin at a 5:1 molar ratio. The PK/PD impact of the CombiPlex approach is readily demonstrated through QWBA images of rats treated IV with VYXEOS compared with conventional administration of the same drugs (Figure 1). When encapsulated within a rationally designed nanoscale carrier designed to coordinate in vivo drug exposure, the QWBA images of cytarabine and daunorubicin for VYXEOS (Figure 5) show significant differences in both biodistribution and pharmacokinetic properties compared to the free drugs. At 1 hour (left side images), both VYXEOS-delivered cytarabine and daunorubicin show less systemic distribution to tissues than the free drugs (Figure 1) and demonstrate preferential localization to the bone marrow (the site of leukemia growth), as evinced by the higher concentrations of both drugs in the femur. In contrast to the observations with the free-drug combination, both drugs in VYXEOS remain present at the same ratio in the images taken at 24 hours and continue to localize within the marrow at the tumor site (Figure 5). Extensive preclinical testing demonstrating drug ratio-dependent efficacy and significant improvements in efficacy for VYXEOS compared to the free-drug cocktail supported the evaluation of VYXEOS in a clinical setting.

Multiple clinical trials have demonstrated the translation of CombiPlex combination development to improved clinical efficacy. Promising anti-leukemic activity was observed in the Phase I trial of VYXEOS in patients with advanced acute leukemias, most with relapsed AML following prior cytarabine and daunorubicin treatment. VYXEOS produced multiple responses (4 out of 12) at sub-MTD doses as low as 32 units/m² (32% of MTD), with 9 responders at or below MTD, 8 of whom had previously received cytarabine and daunorubicin. These encouraging results led to two randomized Phase II trials in which VYXEOS was evaluated against the 7+3 regimen of cytarabine and daunorubicin in newly diagnosed elderly AML patients and against investigators choice salvage therapy in first relapse AML patients. VYXEOS provided improvements in complete remission rate, 60-day mortality and overall survival in subsets of high risk patients. For newly diagnosed secondary AML patients, VYXEOS nearly doubled the median overall survival from 6.3 to 12.1 months (Figure 6). In the first relapse trial, the median overall survival increased from 4.2 months for salvage treatment to 6.6 months for VYXEOS. The survival improvements in high risk patient subsets were statistically significant in both trials.^6,7

The Phase II trial in newly diagnosed elderly AML patients identified the high risk (secondary) AML population as one with a high unmet need for which VYXEOS appeared to provide the largest efficacy improvement and survival benefit. Subsequently, a Phase III trial evaluating VYXEOS versus 7+3 in this population resulted in a 43% relative improvement in CR+CRi rates for VYXEOS (47.7% vs. 33.3%). Final results are expected in the first quarter of 2016. Taken together, the clinical trial results reported to date provide compelling validation of the CombiPlex approach for the rational development of drug combination products.

COMBIPLEX COMBINATIONS ON THE HORIZON

With strong clinical data providing proof-of-principle, CombiPlex was recently expanded to combinations of MTAs using Celator’s hydrophobic prodrug nanoparticle (HPN) delivery technology.⁸ The HPN concept is broadly applicable: MTAs from diverse classes, including inhibitors of MEK, Akt, HSP90, B-Raf, and FGFR as well as docetaxel, were all successfully formulated in polymer nanoparticles. In all cases, HPN delivery eliminated the early distribution phase observed for conventional formulations of these agents, which has been associated with significant exposure and toxicity to normal tissues. This was reflected by the fact that HPN co-formulated combinations of docetaxel plus the HSP90 inhibitor AUY922 as well as the MEK:Akt inhibitor combination of selumetinib plus ipatasertib could be administered at much higher doses.

For both combinations, the CombiPlex formulations provided significant improvements in efficacy over the free-drugs in human xenograft cancer models. Furthermore, evidence of strong drug ratio-dependent efficacy and in vivo synergy was observed in which the largest improvements were observed in models that were more resistant to the drugs administered in their conventional forms. Once HPN formulation conditions were optimized for these first two combinations, the approach was readily extended to the B-Raf and FGFR inhibitors. Both were successfully co-formulated with the MEK inhibitor selumetinib, resulting in coordinated drug exposure in the plasma. In addition, prodrug components could be “mixed and matched.” This versatility was exploited to generate a 3-drug combination, a co-formulation of selumetinib, AUY922, and docetaxel prodrugs in a single HPN that exhibited a plasma half-life of 10 hours in mice with no early distribution phase.

CONCLUSION

The traditional drug development path may limit the ability to capture the full efficacy potential of combinations composed of chemotherapeutics as well as highly potent molecularly targeted agents due to the uncoordinated PK/PD properties of the individual drugs and the presence of drug ratio-dependent synergy/antagonism. CombiPlex provides an avenue to develop and optimize combinations prior to approval of individual agents in a manner that controls drug ratios and coordinates in vivo drug exposure through the use of nanoscale drug carriers. Optimizing combinations as early as possible in the development process may enhance efficacy, improve safety, and reduce the time and patient resources required to create effective combination therapies.

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REFERENCES

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4. FDA approves Cotellic as part of combination treatment for advanced melanoma. http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm471934.htm.
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6. Cortes JE, Goldberg SL, Feldman EJ, et al. Phase II, multicenter, randomized trial of CPX-351 (cytarabine:daunorubicin) liposome injection versus intensive salvage therapy in adults with first relapse AML. Cancer. 2015;121(2):234-242.
7. Lancet JE, Cortes JE, Hogge DE, et al. Phase 2 trial of CPX-351, a fixed 5:1 molar ratio of cytarabine/daunorubicin, vs cytarabine/daunorubicin in older adults with untreated AML. Blood. 2014;123(21):3239-3246.
8. Tardi P, Xie S, Liboiron B, et al. Coordinated delivery of anticancer drug combinations incorporating molecularly targeted agents provides markedly increased plasma drug exposure, decreased toxicity and increased efficacy in preclinical tumor models, AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics, Abstract B34, 2015.

Dr. Barry D. Liboiron joined Celator Pharmaceuticals in 2006, as a Research Scientist in biophysics, and was promoted to direct the department as Director, Biophysical Characterization and Advanced Solutions in 2008. Dr. Liboiron earned his BSc with distinction from the University of Guelph, his PhD degree in Inorganic Chemistry from the University of British Columbia, and completed a post-doctoral fellowship in Bioinorganic Chemistry at Stanford University.

Dr. Arthur C. Louie is Chief Medical Officer at Celator Pharmaceuticals. He is a board-certified Oncologist with more than 25 years of experience in Pharmaceutical Research and Development. Dr. Louie earned his BA in Biology with honors from Haverford College and his MD from New York University. He completed oncology fellowships at the National Cancer Institute and Stanford University.

Dr. Lawrence D. Mayer is the President, Chief Scientific Officer, and Founder of Celator Pharmaceuticals. He has played a lead role in the discovery and development of numerous anti-cancer drugs, several of which achieved market approval. Dr. Mayer has authored over 250 publications and has more than 35 patents either awarded or pending. Dr. Mayer earned his BSc summa cum laude from Wartburg College and his PhD from the University of Minnesota.