The Limitations and Challenges of Cancer Therapies
By Michael A. Evans, BSci MSci
Considerable investment into cancer research has led to the development of remarkable new therapies, which have greatly improved therapeutic outcomes for countless individuals. Until recently, almost all therapies granted clinical approval were small molecules. However, biotechnological advances in the last several decades have allowed development of powerful new classes of therapeutics, including macromolecules, nanoparticles, and even engineered cells.
In this blog post, we explore the advantages and limitations of some of the most common classes of therapeutics: small molecules, macromolecules, nanoparticles, and chimeric antigen receptor T-cells.
Small molecule
Small molecule drugs encompass organic and inorganic compounds typically less than 1 nm in diameter and 100 atoms in size [1,2]. Many of these drugs can be chemically synthesized, which helps with scalability and production costs. In addition, the size of these chemical entities means they are not subject to folding and denaturation constraints like larger macromolecules, simplifying formulation [3,4].
Advantages
These advantages mean 90% of available therapeutics fall under this class [5]. In most cases, small molecule therapeutics are designed to bind to a biological target in a manner that alters its behavior in a desired manner [2,5]. Various small molecules are used to treat both solid and blood cancers. The effects vary based on the drug and can include direct cytotoxicity [6], angiogenesis inhibition [7], and immunomodulation [8], among others.
Disadvantages
While small molecules do provide some benefits to patients, they have many problems that limit their efficacy.
Small Size
The size of these drugs means the available surface area that can interact with a target is small. Thus, binding areas on biological targets must allow for a strong interaction with the drug.
For many proteins, these areas have been elusive, making them “undruggable” targets with today’s small molecules. It is estimated that only 3,000 of the 25,000 total proteins in the human genome contain areas suitable for binding by small molecule therapeutics [2].
Issues with Tissue Transport
Many small molecules also have undesirable tissue transport abilities. A drug must passively diffuse to a target tissue after leaving the bloodstream. Reducing vascular density inside solid tumors increases the distances these drugs must travel to ensure full coverage [9,10].
For many drugs, their diffusion rate and tumor penetration is dictated by their hydrophobicity [10]. However, reducing the polarity of these drugs can inhibit their aqueous solubility. Hydrophilicity is essential for formulation development and is a significant problem for up to 40% of newly identified drug candidates [11].
Short Half Lives
In addition to diffusion limitations, many (but not all) small molecules have short biological half-lives [12]. This issue is largely due to metabolic processing in the liver and excretion by the kidneys leading to frequent administration to maintain a therapeutic effect [13,14].
Frequent doses are problematic because they reduce the product's drug concentration and lower the likelihood of patient compliance [14].
Macromolecules
Macromolecules such as nucleic acids, proteins, and carbohydrates are common biological building blocks often developed as therapeutic entities. Since the approval of recombinant human insulin 36 years ago, advances in biology have made the development of macromolecular drugs more economical [15]. Because of this, 28.8% of drug approvals in 2018 were for macromolecular therapeutics [16]. The sheer variety of drugs in this area is too vast to describe each in detail. Instead, we will focus on antibodies, one of the most successful subsets.
Antibodies
Antibodies are one of the most popular classes of therapeutics. Revenue from these drugs represented 50% of the $140 billion in profit reported by the pharmaceutical industry in 2013 [17]. Antibodies are large Y-shaped proteins with a molecular weight of around 150 kDa [18]. The immune system uses the proteins to recognize pathogenic targets, among other things.
These proteins contain two important regions. At the base of the Y is the fragment crystallizable region (FcR). The structure of this region remains consistent between antibodies and allows immune cells to recognize the antibodies present. The two tips of the Y are known as the antigen-binding fragments (Fab).
These areas differ between different antibodies, and their structures are optimized to allow specific binding to a target antigen [18]. Unlike small molecules, antibodies' large size and folding constraints require the assistance of the protein assembly machinery inside living cells for their industrial production [18]. While there are several classes of antibodies, those currently in the clinic all belong to the immunoglobulin G (IgG) family [18].
Therapeutic Effects
Like small molecules, the therapeutic effects of antibodies are diverse and include immunomodulation [17], angiogenesis inhibition [7], and direct tumor cell cytotoxicity [17]. In addition, an effort has been devoted to developing antibodies with therapeutic small molecules conjugated to them to improve the targeting of the latter [17].
Because of their size, antibodies often have a much higher affinity and specificity for their target than small molecules. This higher binding affinity allows antibodies to target many structures that small molecules cannot [2]. Because antibodies are naturally present in the body, they often exhibit superior circulation half-lives (up to 21 days), reducing the administration frequency [18,19].
Despite being more costly, antibodies' advantages mean that 18-24% of antibodies that enter clinical trials are approved compared to the 5% seen with small molecules [14].
Drawbacks
While antibodies have some significant advantages, they also have some drawbacks. Like small molecules, antibodies use passive diffusion to distribute in the tissue. However, their larger size can make this even more difficult.
In addition, the high binding affinity of many antibodies means they can associate with antigens near the blood vessel, which lowers their penetration into deeper areas of the tumor [19]. These factors have led to only 20% of injected antibody doses being able to interact with the tumor despite a long half-life [19].
This is such an issue that in 2009 only 2 of the 9 antibodies approved for cancer therapy were for solid tumors, even though 85% of all human cancers are of this type [19]. Additionally, unlike small molecules, antibodies, and other macromolecular therapeutics usually cannot be designed to pass through the cell membrane due to their large size14. This limits these therapeutics to less than 10% of targets on the cell surface [2].
The formulation of antibodies is much more challenging than small molecules. The large size of these macromolecules means they can fold into different tertiary structures, only one of which will produce the desired therapeutic effect. Small changes in pH, ionic strength, and temperature can encourage denaturation of these macromolecules and complete activity loss. In addition, larger molecules can run into physical issues such as high formulation viscosity, which makes their administration infeasible [3].
In addition to formulation constraints, tumors can resist these antibodies through multiple escape mechanisms. Because tumors are phenotypically heterogeneous, there may be populations of cancer cells that lack expression of the target antigen. These cells can repopulate the tumor, allowing it to recur in a form resistant to the antibody [20]. Heterogeneity in receptor expression has been reported to be especially common between primary tumors and metastatic lesions [21,22].
Nanoparticles
Nanoparticles are drug-delivery vehicles developed to address many of the issues faced with conventional therapeutics [23]. In preclinical and academic environments, this approach has been able to improve drug solubilization of hydrophobic drugs [11], control drug release [23], and allow for the delivery of multiple therapeutic entities in a single package [24].
These advantages have led to increased research in this area, with over 10,000 papers published on nanoparticles yearly since 2011 [25]. Despite the intensive level of research, only a handful of nanoparticles have made it to the clinic [26,27]. This lack of translation stems from issues that have yet to be effectively addressed.
Challenges
The size of particles and the materials used to make them often lead the body’s reticuloendothelial (RES) system to view them as foreign and potentially hazardous objects, which leads to their rapid clearance from circulation by the liver and spleen [23]. This expedited removal from circulation reduces the number of passes nanoparticles can make through a diseased tissue, which lowers the accumulation of particles at the desired site.
Many strategies to circumvent this issue have been devised [23]. One of the most popular methods to improve nanoparticle circulation is grafting Polyethylene glycol (PEG) to the nanoparticle surface. This material has been demonstrated to act as a particle camouflaging agent, improving the circulation time of nanoparticles in vivo.
However, its popularity as a chemical additive in cosmetics has led to the development of PEG antibodies in 25% of the human population [28]. In addition, particles that can accumulate on target do not penetrate very deeply into the tissue.
One study found that DOXIL, a clinically approved nanoparticle containing doxorubicin, could only penetrate 34 µm away from the nearest tumor blood vessel. This is a considerably shorter distance than the 78 µm doxorubicin could travel on its own [29].
EPR Effect
Cancer has been an area of focus for nanoparticle-based drug delivery. For several years, it was hypothesized that tumor vasculature was leaky, allowing nanoparticles to preferentially extravasate into cancerous tissue. Known as the enhanced permeation and retention (EPR) effect, this hypothesis was one of the primary drivers behind using nanoparticles to treat cancer [30].
Unfortunately, recent evidence indicates that the benefits of the EPR effect are exaggerated [27,30]. Reports by others studying nanoparticle accumulation in mice that were intravenously injected with nanoparticles of various chemical and physical properties showed that, on average, only 0.7% of the injected dose was found in these tumors.
Perhaps most alarmingly, this value has remained constant for the last decade, indicating that our intensive study of these delivery systems has failed to help overcome one of their root causes of clinical failure [27].
Chimeric Antigen Receptor (CAR) T-Cells
CAR-T are a form of gene-modified cell therapy that was recently approved by the FDA, leading to an explosion of interest in the cell therapy arena. Much of the clinical success of CAR-T is due to the ability of the CAR to leverage the powerful cancer cell-killing machinery inside T-cells [31].
Initial results with CD19+-targeted CAR-T are exciting and led to remission rates of 83% in patients with relapsed or refractory acute lymphoblastic leukemia [32]. In addition, CAR-T-based therapies demonstrate lower rates of side effects [33], long-term persistence [34], and higher survival rates [34] in comparison to other types of therapies.
Potential Problems
While the future of CAR therapies looks very promising, issues still limit their clinical potential. One issue that CAR-T cells face when treating solid tumors is penetration. At first glance, the penetration issues seen with CAR-T cells mirror those seen with the other therapies discussed.
However, the mechanism by which penetration is achieved is quite different. Small molecules, macromolecules, and nanoparticles are man-made structures that lack effective active mechanisms for targeting and penetrating diseased sites. These complexes rely heavily on diffusion and convection to distribute them in the tumor [9].
The large size of T-cells means that tissue distribution via diffusion is out of the question. Because of this, T-cells and other immune cells have been bestowed with chemotaxic properties. Chemotaxis allows these cells to sense chemical gradients from inflammation and apply mechanical forces to leave the bloodstream and squeeze their way between cells in tissue to reach an inflamed area [36]. The poor penetration of CAR-T cells into tumors is due to their ability to suppress CAR-T chemotaxis through processes such as the excretion of CXCL12 and CXCL537.
CAR-based therapies also face production difficulties. Current systems utilize viral vectors to allow for CAR expression in the cells. Unfortunately, viral vectors are expensive and can produce gene silencing and oncogenic side effects [42]. Thus, widespread adoption of CAR therapies will require other approaches.
Possible Solutions
However, circumventing these suppressive mechanisms through further genetic modifications could rapidly improve CAR-T penetration into solid tumors [37]. Another approach to overcome this hurdle would be using a different immune cell type. For example, macrophages have been shown to accumulate in large numbers in solid tumors, and incorporating a CAR seems to turn macrophages into potent tumor killers [38-41].
The targeting abilities used by CAR-T cells to specifically identify and destroy cancerous cells means they have a lower chance of side effects [33]. However, the power and long-term persistence of these cells mean that the side effects that do occur are often more severe.
One of the most significant complications from CAR-T therapy is cytokine release syndrome (CRS) due to off-target CAR-T activation. Induction of CRS leads to systemic inflammation that can be life-threatening. Current research efforts to eliminate this issue are underway and include strategies such as the incorporation of suicide genes and improvement in on-target activation [33].
Mainstream non-viral methods such as electroporation are plagued by poor transfection efficiency and/or high cell toxicity [43,44]. We believe the microfluidic vortex shedding method used by the Indee hardware provides a non-viral solution that can efficiently transfect cells without the disadvantages seen with viral and non-viral methods [44].
In Conclusion
Today’s rapid expansion of the therapeutic landscape with a widening array of different technologies will only help to expand the options scientists have for developing treatment modalities resulting in a more rapid pace of pharmaceutical innovation. Here, we reviewed the efficacy of small molecules, macromolecules, nanoparticles, and CAR-T cells as cancer therapies.
Each of these therapeutic classes comes with advantages and disadvantages that must be considered when designing a new drug.
· Small molecules can often be produced through chemical synthesis and are not subject to denaturation, which can simplify production and formulation while reducing costs. However, these molecules are limited by elusive binding sites on many biologic targets, tissue transport, and half-life constraints.
· Macromolecules such as antibodies benefit from their high target specificity and long circulation times but face challenges such as denaturation and poor tumor penetration.
· Nanoparticles have the potential to improve drug solubility, control drug release, and deliver multiple drugs in the same package. Unfortunately, rapid clearance by RES organs and poor tumor targeting have led to a disproportionately small number of nanoparticles reaching the clinic.
· CAR-T cells are one of the most recent additions to the therapeutic toolbox and have shown remarkable success because of long-term therapeutic persistence, lower side effects, and higher survival rates than other therapies. Currently, these cells face several obstacles that have hindered their clinical translation to solid tumors, including poor penetration, functional suppression, cytokine release syndrome, and costly production.
However, the relative novelty of CAR-T compared to these other types of therapies means that many of these challenges may soon be overcome, making it and other cellular therapies increasingly attractive soon.
About the Author
Michael A. Evans, BSci MSci is a bioengineering Ph.D. candidate at Harvard University with 7 years of experience in immunology, cell therapies, drug delivery, and organic chemistry. His current research focuses on using macrophages as carriers for nanoparticles to improve their targeting towards inflamed tissues. He is an author of 4 peer reviewed publications with three more in press. He attended Furman University and the University of California, Santa Barbara (UCSB), where he received a B.S. and M.S. in chemistry, respectively. He received the 2014 John Sampey Award for the Chemical Sciences and was a 2017 UCSB New Venture Competition Finalist and the People’s Choice Award recipient. Michael will be finishing his PhD in late 2019 and is excited to begin his career in gene-modified cell therapy.
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