The pharmacokinetic and pharmacodynamics properties of cancer therapies within the circulation have been extensively studied in order to balance the therapeutic efficacy of a drug with its toxicity; however, drug levels and their effects on cancer cells are profoundly altered by the local tumor microenvironment (TME). The TME is shaped by the presence or absence of essential nutrients, concentrations of signaling molecules, and nearby, nonmalignant cells. This chapter focuses on the impact of the TME on the pharmacokinetics and pharmacodynamics of cancer therapeutics.

Figure 8-1

Depending on their specific chemical properties, each drug is expected to have a unique volume of distribution and tissue penetration. In solid tumors, for instance, chemotherapy may reach therapeutic concentrations within the organ of origin, and metastasis to another organ, perhaps less permeable to chemotherapy, provides the tumor with a mechanism of escape. Similarly, for liquid tumors, such as acute leukemia, circulating blasts may be exposed to drug pharmacokinetics significantly different than malignant cells residing in various bone marrow microenvironments.

Mechanisms that have a profound impact on drug pharmacokinetics in the TME include:

1. Limited blood flow: Poor blood flow not only reduces delivery of drugs to the TME, but also produces a hypoxic and acidic environment, which may further impair drug delivery. An acidic environment favors protonation of basic drugs, resulting in low cellular uptake. Cancer cells preferentially reside within supportive niches and actively sculpt the local vasculature to achieve an even more favorable microenvironment. For example, the bone marrow has several distinct niches, including the vascular and endosteal niches, which differ in terms of blood flow, pH, and oxygen tension. Leukemic cells anchored within the endosteal niche may be less exposed to therapeutic agents and are responsible for residual disease upon initial treatment. Heterogeneity of microenvironments within the tumors themselves allows the cancer to diversify its portfolio—benefiting from high blood flow environments when nutrients are rich and from low blood flow environments when toxic drugs are in circulation. The presence of necrosis within a tumor exemplifies this trade-off; tumor cells in necrotic regions grow slowly or not at all, but are shielded from the high concentrations of toxic drugs that would otherwise lead to their demise.

   Figure 8-2

2. Expansion of the extracellular matrix (ECM) and stromal compartment: Includes a diversity of ECM components such as collagen, hyaluronan, fibronectin, and laminin, which increase interstitial pressure and produce a physical barrier that reduces diffusion of drugs throughout the tumor.
3. TME expression of drug-metabolizing enzymes: Drug metabolism at the level of the TME creates unique local drug concentrations. Tumor cells and the surrounding stroma upregulate cytochrome P450 (CYP) enzymes in response to drug exposure, resulting in increased metabolism and locally reduced drug concentrations. CYP3A4, in particular, is responsible for the metabolism of a vast array of oncologic therapies, including topoisomerase inhibitors, vinca alkaloids and proteasome inhibitors, and also nearly all small-molecule inhibitors, most notably the large family of tyrosine kinase inhibitors. Similarly, mesenchymal stroma cells express high levels of cytidine deaminase (CDA) that can inactivate cytarabine and azacitidine, which are two active drugs in acute leukemia and myelodysplastic syndrome.
4. Expression of efflux pumps: Although expression of efflux pumps is not the focus of this chapter, it is worth noting that the drug concentrations in the TME may differ yet again from those in the tumor cells themselves. Although there are multiple mechanisms that can account for potential differences, perhaps the best characterized and most noteworthy is the overexpression of drug efflux pumps such as ATP-binding cassettes (ABC) transport proteins, which actively pump deleterious drugs out of the tumor cells.

Understand ing TME-dependent mechanisms of impaired local pharmacokinetics may provide opportunities for novel therapeutic strategies. For instance, drugs that are repackaged within liposomes or nanoparticles have altered volumes of distribution and tissue penetration, favoring uptake within the TME. These larger drug particles preferentially extravasate into tumor sites that are supplied by fragile and relatively leaky vasculature. Liposomal doxorubicin as well as the liposomal combination of daunorubicin and cytarabine are two illustrative examples. The effectiveness of liposomal doxorubicin is dramatically improved in the treatment of patients with Kaposi’s sarcoma affecting the skin due to improved tumor penetration and retention. The liposomal formulation also reduces deposition in unaffected organs such as the heart, reducing adverse effects. The liposomal formulation of daunorubicin and cytarabine not only optimizes the delivery of both drugs into the tumor but may also ensure pharmacokinetic synchronization of the fixed 5:1 molar ratio to increase synergism and their uptake by leukemia cells. Another promising strategy is the coadministration of drugs that block drug-metabolizing enzymes such as CYP3A4. Clarithromycin is a potent CYP3A4 inhibitor and is used in combination with lenalidomide and dexamethasone (BiRD regimen) to optimize the pharmacokinetics and local effects of dexamethasone, resulting in improved responses compared to lenalidomide and dexamethasone alone. More so, a clinical trial of clarithromycin and cabazitaxel aims to test the efficacy of this same strategy in the treatment of metastatic, castration-resistant prostate cancer.

Some of the mechanisms by which TME changes the responsiveness of malignant cells to chemotherapy include:

1. Providing antiapoptotic survival and growth signals: Cancer cells are involved in a complex crosstalk with nonmalignant cells within the TME. Nonmalignant cells often elaborate prosurvival and growth signals that fortify malignant cells and attenuate the effects of the drug on the tumor, even at therapeutic concentrations. Leukemia cells, for example, overexpress a variety of receptors, including c-kit, Flt3, and AXL, which are typically expressed by hematopoietic stem cells. Activation of these receptors by signaling molecules produced by mesenchymal stromal cells leads to upregulation of antiapoptotic signaling programs that decrease the efficacy of chemotherapy.

   Figure 8-3

2. Modulating differentiation state as a means of avoiding drug effect: Tumors contain a heterogeneous population of subclones, which differ in their state of differentiation. In the case of multiple myeloma, mesenchymal stroma cells promote an immature phenotype of malignant plasma cells characterized by decreased immunoglobulin production, low endoplasmic reticulum (ER) stress, and , resistance to proteasome inhibitors. In fact, myeloma cells reinforce this protective mechanism by secreting hedgehog ligand s that upregulate stromal CYP26 and create low retinoid niches where immature malignant cells can survive.
3. Maintenance of quiescence: The toxicities of many cancer therapies are dependent on active cell cycling. These include cytotoxic chemotherapies and CDK inhibitors. Cancer cells escape toxicity by maintaining a quiescent state. The TME is a crucial mediator of quiescence. Hypoxic environments lead to upregulation of hypoxia inducible factor (HIF)-1α, which, in turn, maintains cells within a quiescent state. Adhesion of tumor cells to specific components of the ECM is necessary for proliferation. Laminin-rich ECM influences gene expression within mammary epithelial cells to produce a script that favors quiescence and suppression of apoptosis.
4. Production of essential nutrients: Upregulation of nutrient production within the TME to overcome therapies aimed at depleting these same nutrients. Pegasparaginase is a pegylated enzyme that is used in the treatment of acute lymphoblastic leukemia and functions by depleting asparagine, a nonessential amino acid that is required for leukemic proliferation. Malignant cells are capable of inducing local mesenchymal stromal cells to overexpress asparagine synthetase in order to abrogate the effects of systemic asparagine depletion.
5. Vasculature: Cancer cells stimulate angiogenesis within the TME through the production of cytokines, such as vascular endothelial growth factor (VEGF), in order to increase delivery of nutrients necessary for proliferation. VEGF inhibitors, such as bevacizumab, and vascular endothelial growth factor receptor (VEGFR) inhibitors, such as sunitinib or sorafenib, block angiogenesis.
6. Anchoring: Tumor cells benefit from the TME in myriad ways, not least of which is the protection it provides cancer cells from existing therapies. Cancer cells anchor themselves to the TME via interactions with the ECM, the endothelium, and other inhabitants of the TME. For example, leukemic and prostate cancer cells express the CXCR4 receptor, which is responsible for homing and anchoring hematopoietic stem cells to the marrow.

In conclusion, interfering with these TME-dependent mechanisms holds promise to positively impact tumor stem cells and sensitize them to chemotherapy. To this end, tyrosine kinase inhibitors such as midostaurin and sorafenib inhibit activation of c-KIT and FLT3 to dampen survival and proliferation signals. A variety of new FLT3-specific and AXL inhibitors are being investigated as potential therapeutic options in clinical trials (e.g., NCT02488408).

Drugs that induce differentiation, such as retinoids, DNA methyltransferase inhibitors such as azacitidine, and IDH1/2 inhibitors, reduce tumor stem cells’ inherent hardiness and resistance to apoptosis by differentiating them into more vulnerable progenitor cells. The use of CYP26-resistant retinoid either as single agent (IRX5183—NCT02749708) or in combination with azacitidine (SY-1425—NCT02807558) holds promise to bypass the biochemical barrier imposed by mesenchymal stroma and differentiate leukemia stem cells in the niche.

Finally, tumor stem cells can be mobilized out of their niche in an effort to not only expose them to systemic drug pharmacokinetics but also interrupt the direct effects of their TME. To this end, G-CSF and Plerixafor can effectively mobilize malignant cells in patients with hematological malignancies and solid tumors. To what extent this strategy sensitizes cancer cells to chemotherapy and improves clinical outcomes remains to be further explored.

The tumor microenvironment plays a critical role in modulating the impact of anticancer therapies on their targets. The TME contains multiple components that interact to influence net drug delivery to tumor cells and to modulate the net balance between tumor cell proliferation, differentiation, survival, and death. The TME is capable of reducing the cancer stem cells’ exposure to anticancer agents and can attenuate their toxic effects through paracrine signaling and cell–cell interactions. Through better understand ing of the interactions between tumors and their environments, we may be able to enhance the effectiveness of existing cancer treatments and to identify new targets for anticancer therapeutics. Successful differentiation therapy is perhaps best exemplified by the use of all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) in acute promyelocytic leukemia (APL, M3-AML) and reflects the impact of tumor-directed therapies on both the TME and the malignant stem cell.

• “Differentiation syndrome” (DS) caused by both ATRA and ATO therapies for APL consists of hyperleukocytosis and cytokine release syndrome with pulmonary and renal dysfunctions. DS is successfully prevented and /or managed in high-risk APL patients with high-dose steroids and hydroxyurea.
• IDH1 and IDH2 inhibitors are also associated with a syndrome of differentiation that is clinically similar to that seen in APL. This syndrome can occur both early (e.g., days 3–7) and late (days 25–35) during therapy and is managed with high-dose steroids and hydroxyurea.
• Anti-angiogenesis agents targeting VEGF isoforms and receptors are characteristically associated with hypertension and proteinuria. In addition, tumor necrosis at specific organ sites can lead to hemorrhage and /or perforations in the brain, lung, or gastrointestinal tract.
• Bevacizumab (Avastin), a VEGF-A–directed monoclonal antibody, is approved for use as a single agent or in combination with other antitumor modalities for melanoma, glioblastoma, and renal cell, ovarian, cervical, non-small cell lung, and colorectal carcinomas.
• ATRA can increase intracranial pressure, causing pseudotumor cerebri associated with headache, papilledema, and sensorineural hearing loss, and is managed with high-dose steroids and acetazolamide.
• To prevent cardiac complications of arsenic trioxide (ATO), patients must have adequate plasma potassium levels (>4) and magnesium levels (>2), and QT< 500 ms during each ATO administration. Agents that prolong QT should be avoided or used sparingly.

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