US Pharm. 2008;33(10)(Oncology suppl):3-14,23.

ABSTRACT: Tyrosine kinases are a family of proteins that contribute to the development of cancer. Anticancer drug development has recently taken aim at these receptors. The tyrosine kinase inhibitors (TKIs) are a class of small-molecule, orally administered agents with a unique mechanism of action. Since these drugs are administered orally, they can be dispensed in any practice setting. The purpose of this article is to provide an overview of TKIs and review important considerations for dispensing these agents.

Traditionally, the pharmacologic management of cancer utilized primarily IV medications and was therefore dismissed as having little relevance to the community pharmacist. However, recent developments in the field of targeted therapy are starting to erode this paradigm. Since the start of the new millennium, small-molecule, orally administered drugs that target specific tumorigenic proteins have been available to treat cancers. These tumorigenic proteins, known as tyrosine kinases, can be subdivided into two broad classes based upon their structure, function, and localization. Both receptor tyrosine kinases (RTKs) and nonreceptor tyrosine kinases (NRTKs) have been implicated in the development of multiple types of cancer including, but not limited to, leukemia, lung cancer, breast cancer, pancreatic cancer, and gastrointestinal stromal tumors (GISTs). Agents that target these proteins have a number of distinct advantages over conventional chemotherapy including a reduction in systemic toxicity and the ability to be administered orally.

As additional oral, small-molecule targeted therapies become available, the community pharmacist will become more involved in the care of cancer patients. These patients will expect their pharmacists to provide counseling regarding these new drugs. Therefore, pharmacists should, at minimum, understand the pathobiology, pharmacology, indications, side effects, and drug interactions of these agents.

Pathobiology
Receptor Tyrosine Kinases: RTKs are a superfamily of cell membrane proteins that possess common structural features to include an extracellular ligand-binding domain, a transmembrane region, and a cytoplasmic region containing adenosine triphosphate (ATP)–binding and enzymatic kinase domains (FIGURE 1). There are approximately 60 known and characterized RTKs that are divided into at least 20 subfamilies based on similar receptor characteristics and/or common ligands.1 These proteins are critical in capturing and transducing extracellular signals carried by peptide-based ligands, referred to as growth factors. Their signals help regulate normal cellular processes associated with cell life span, cellular proliferation, and differentiation.



In their inactive state, RTKs exist as monomeric transmembrane proteins (FIGURE 1A). Once activated, these proteins dimerize and form oligomeric pairs. The formation of receptor oligomers is coupled to the activation of the receptors' enzymatic domains and autophosphorylation of tyrosine residues contained within the intracellular domain of the receptor (FIGURE 1B). Phosphorylation of tyrosine residues on the receptor and effector proteins occurs when ATP binds to a specific region of the receptor. Once bound, the receptor removes a phosphate group from ATP and transfers it to a tyrosine residue on either the receptor or effector protein. Phosphorylated tyrosines on the receptor are thought to serve as docking sites for a variety of effector proteins that participate in multiple signal transduction cascades coupled to these receptors. Once docked, these effector proteins can be activated by the receptor through additional phosphorylation reactions (FIGURE 1C).2,3

It is evident that ATP binding is a critical component of RTK activity. If the ability of these receptors to bind and utilize ATP is impaired, their activity will be greatly diminished. This serves as a key component of targeted therapy activity.2,3

While RTKs play an important role in the normal regulation of many cellular processes, when abnormalities occur in their expression (i.e., autoregulatory mechanisms, intracellular signaling, or responsiveness to extracellular ligands), they can cause cells to divide uncontrollably, thus participating in the pathobiology of cancer. It is now apparent that the activity of a specific subclass of RTKs (subclass 1 or ERBB) is abnormal in many types of epithelial cancers. There are four members of the ERBB subclass: ERBB1, ERBB2, ERBB3, and ERBB4. The nomenclature for these receptors can be confusing, as there are multiple designations for each receptor subtype, and all are used interchangeably in the literature. For example, the ERBB1 receptor is also commonly referred to as the epidermal growth factor receptor (EGFR) or the human epidermal growth factor receptor (HER1). The ERBB2, ERBB3, and ERBB4 receptors also have multiple designations, commonly referred to in the literature as EGFR2, HER2/neu, HER3, and HER4, respectively.

Other RTKs that appear to be important in tumor development include vascular endothelial growth factor receptors (VEGFR, VEGFR2) and platelet-derived growth factor receptors (PDGFRs). These receptors have been identified as important mediators of blood vessel growth into tumors (angiogenesis), as well as promoters of tumor metastases. VEGFRs and PDGFRs have a number of basic similarities to ERBB receptors with respect to activation, oligomerization, and autophosphorylation.4 Overexpression of these receptors and/or their associated ligands has been linked with increased vascularization of solid tumors, an increase in the recurrence of cancers, and a decrease in patient survival.5

In addition, the FMS-related tyrosine kinase-3 receptor (FLT3) is expressed by early hematopoietic progenitor cells and plays an important role in the development of these cells. Mutations in FLT3 can cause constitutive activity, contributing to disregulated division of hematopoietic progenitor cells and leading to the development of acute myelogenous leukemia.6

With the overwhelming evidence that this family of receptors is so important in the pathobiology of cancer, it is not surprising that developing compounds to inhibit these receptors has become a major focus in cancer therapy research.

Nonreceptor Tyrosine Kinases: NRTKs are a diverse group of cytosolic proteins found in various regions of the cell, including the inner surface of the plasma membrane and nucleus. Like their membrane counterparts, these proteins play an important role in regulating cell proliferation, differentiation, metabolism, migration, and survival by participating in cellular signaling cascades that are activated by a variety of signals to include hormones, neurotransmitters, growth factors, and cytokines. Currently, there are nine families of NRTKs, with multiple members in each family. They are the ABL (Abelson), Src, Tec, CSK, FAK, SYK, JaK, TnK, and FeS families.7 Functionally, each class of NRTKs works by catalytically transferring a phosphate group from ATP to a tyrosine residue on an effector polypeptide/protein. As with the RTKs, the transfer of a phosphate group from ATP to an effector protein is an important component for regulating the activity of signaling cascades that help govern cellular processes such as proliferation and differentiation.

Given the role of NRTKs in cellular function, it is not surprising that their activity is kept under tight control by the cell. Like their receptor counterparts, when these proteins cease to function normally due to genetic mutation, leading to abnormal signaling, loss of autoregulatory processes, or overexpression, they can participate in the pathology of many types of cancer.8 Therefore, it is not surprising that this group of proteins has also become an important therapeutic target for the treatment of neoplastic disease.9

One NRTK in particular, c-ABL, has been studied extensively and been shown to play an important role in the development of chronic myelogenous leukemia (CML). Normally, c-ABL participates in a number of cellular processes, including regulation of the cell cycle.10 However, in CML patients, a chromosomal abnormality is present that alters the structure and activity of this NRTK. Known as the Philadelphia chromosome (Ph), this defect occurs when pieces from two chromosomes, 9 and 22, which contain the genes for c-ABL and breakpoint cluster region (BCR), respectively, become translocated and fused together. The resulting hybrid, the BCR-ABL oncogene, produces an NRTK that has the propensity to oligomerize, becoming hyperactive and unregulatable.11 This genetic abnormality is seen in 95% of patients with CML and between 15% and 30% of patients who have acute lymphoblastic leukemia.12,13

Pharmacology
Small-molecule tyrosine kinase inhibitors (TKIs) are a group of orally available compounds that selectively target and inhibit RTKs and NRTKs. These therapies have significant advantages over traditional chemotherapy in that they target specific proteins that are known to have important roles in tumor growth and progression. Because of their pharmacologic specificity, these agents tend to preferentially impact tumor cell function, thus sparing normal cells and reducing much of the systemic cytotoxicity associated with more traditional agents.14 Furthermore, because of their unique toxicity profiles, these agents can be used in conjunction with radiation therapy and more traditional cytotoxic agents to improve the anticancer activity of a given therapeutic regimen. The primary mechanism of the small-molecule TKIs is to inhibit abnormal signals generated by RTKs and NRTKs that lead to the formation of neoplastic cells. Because of their unique chemical structures, these compounds are able to bind to and block the ATP-binding sites on tyrosine kinases. With an impaired ability to bind ATP, the kinase activity of these proteins is reduced, and they are unable to phosphorylate tyrosine residues located on their cytoplasmic domains and effector proteins. Once inhibited, the neoplastic cells may stop replicating abnormally and/or undergo apoptosis (programmed cell death).

Growth factor signaling through RTKs can activate multiple signaling pathways that are interconnected through common effector proteins and/or the activation of one effector protein by another in a separate signaling cascade. This "crosstalk" between receptors and signaling pathways can make inhibition of a single receptor or effector protein therapeutically irrelevant if an accessory pathway can continue to generate and carry the cellular signals that are responsible for abnormal cellular proliferation. Additionally, some tumors are dependent on multiple defects in receptor and enzyme signaling. Drugs that can inhibit the proteins responsible for these defects may work better in some cancers than in those that are more selective. Therefore, depending on the pathology of the cancer in question, the selectivity and specificity of a TKI may be an important determinant of its therapeutic usefulness. For example, in cancers like CML, neoplastic cell development is highly dependent on one major genetic defect, the BCR-ABL oncogene. In this case, a compound like imatinib, which is highly selective for the BCR-ABL tyrosine kinase, is very effective. This is a practical example of the biological phenomenon known as oncogene addiction.15

In contrast, many types of solid tumors are only partially responsive to tyrosine kinase inhibition. The pathology of these types of cancers may be multifactorial and caused by intracellular signaling abnormalities in more than one receptor and transduction pathway and, as such, may not respond to therapies that antagonize the activity of only one cellular protein.16 Some compounds (e.g., lapatinib, erlotinib) are very specific and selective for only one kind of RTK, while some of the newer agents on the market (e.g., dasatinib, nilotinib, sunitinib) are relatively nonspecific and can inhibit multiple RTKs.

Resistance to small-molecule therapy is a phenomenon that has been well documented and shares many similarities with mechanisms of resistance to traditional chemotherapy. These include mutations to target proteins that impair drug binding, increased ability to extrude drug from the cytoplasm, an increased reliance on a secondary signaling pathway that continues to support abnormal growth or proliferation, and permanent activation of downstream signaling molecules that are unaffected by inhibiting proteins higher up in the signal transduction cascade.

An excellent practical example of tumor cell resistance to small-molecule TK inhibition is the development of imatinib resistance in CML. In CML, imatinib is first-line therapy, and resistance to therapy can develop. This drug inhibits the BCR-ABL tyrosine kinase, leading to an inhibition of cellular proliferation and an induction of apoptosis. Resistance to therapy can develop in patients who have been receiving imatinib therapy for several years or where relapse occurs; the leukemia cells often express a mutated form of the BCR-ABL that is resistant to imatinib inhibition. The specific mutation lies in the ATP-binding pocket near the catalytic domain, and because of its strict binding requirements, this affects the ability of imatinib to inhibit the activity of BCR-ABL. Fortunately, another small-molecule inhibitor, dasatinib, because of its less stringent binding requirements, is still able to bind to the ATP-binding pocket of the mutated form of BCR-ABL and inhibit it. Another way in which resistance develops is that tumor cells often have the ability to switch from one receptor signal transduction pathway that is being inhibited to another, which will continue to support abnormal proliferation and survival.17

Indications
Currently, the TKIs are FDA approved to treat breast, lung, pancreatic, and kidney cancer, as well as GIST (TABLE 1).18-24 Lapatinib is an inhibitor of HER2 (ERBB2) and is used for the treatment of breast cancer in combination with capecitabine.18 Erlotinib and gefitinib inhibit EGFR-1 (ERBB1) and are FDA approved for the treatment of non-small cell lung cancer (NSCLC) after failing chemotherapy.19,20 Erlotinib also has an indication for the treatment of pancreatic cancer in combination with gemcitabine.19 Based on revised labeling, gefitinib's use is limited to patients who have previously taken the drug and are benefiting or have benefited from it.20 The BCR-ABL inhibitors imatinib, dasatinib, and nilotinib are used to treat CML.21-23 Interestingly, these drugs also inhibit the cytokine receptor c-KIT and display activity against GIST. Sunitinib is indicated for the treatment of advanced renal cell carcinoma and GIST.24 Most of these agents are also undergoing investigations for other cancers, such as colon and head/neck.25

The TKIs currently available commercially are somewhat selective in their activity, in that they mostly inhibit one or two TK receptors. However, the TKIs in the current development pipeline tend to inhibit many different types of tyrosine kinases. These "multikinase" inhibitors represent the second generation of TKIs that would theoretically have a broader spectrum of activity and would hopefully be used to treat various types of cancer.



Clinical Data and Applications
As a class, the TKIs have relatively limited data compared to most other cytotoxic chemotherapy used for the same cancer. Therefore, most of the clinical data currently available demonstrate their use primarily in the metastatic stage of cancer. For example, erlotinib currently only has data demonstrating response in metastatic lung and pancreatic cancer, while lapatinib's results are for metastatic breast cancer. Generally, the results in clinical trials have not been particularly impressive (i.e., erlotinib improves overall survival by 2 months and lapatinib delays progression by 4 months).26,27 Nonetheless, these numbers can be clinically significant to a cancer patient or family members. Moreover, these oral agents can be attempted as first-line therapy for patients who cannot tolerate chemotherapy because their poor performance status would leave them susceptible to chemotherapy morbidity and mortality. As we develop more experience with this class of drugs, their use may expand into the adjuvant setting, perhaps even as part of a chemotherapy regimen.

One exception to this last-line rule is imatinib's role in treating CML. Because imatinib demonstrates excellent long-term response rates along with minimal toxicity compared to the alternative (i.e., interferon, chemotherapy), it is the recommended first-line agent to treat chronic phase CML.28,29 There are some patients who have disease resistant to imatinib. In these situations, dasatinib and/or nilotinib are used to overcome imatinib-resistant disease.

Adverse Effects
It is important to note that TKIs are not cytotoxic chemotherapy and, therefore, do not exhibit the worrisome adverse effects of myelosuppression, hair loss, kidney damage, or peripheral neuropathy. Instead, these drugs are touted as being "well tolerated" by many practitioners. This term should not be misconstrued as meaning "the absence of side effects." Rather, it is a statement of comparison to the aforementioned effects of chemotherapy.

Rash: An erythematous, maculopustular rash (also referred to as an acneiform rash) is an important adverse effect associated with the EGFR1 inhibitors erlotinib and gefitinib. This rash commonly presents on the face, neck, and trunk area. Although not life threatening, it is uncomfortable and can be disfiguring. On a positive note, the rash is correlated with response to therapy in clinical trials. Therefore, it can be a reassuring sign to patients that their therapy is working. Mild skin eruptions may be treated with OTC topical antiacne medications, while more severe cases may require treatment with oral antibiotics (e.g., minocycline or tetracycline). If the rash is extremely severe and disfiguring, the offending drug may need to be discontinued or held.30

Diarrhea: In clinical trials, diarrhea was a common adverse effect with all of the TKIs, with an incidence of up to 50% of patients.19 The diarrhea is unlikely to resolve on its own. Loperamide may be used to control these symptoms. Alternatively, a reduction in the TKI dose may be considered.

Interstitial Lung Disease: Perhaps the most dangerous adverse effect with the EGFR TKIs erlotinib and gefitinib is the risk for interstitial lung disease (ILD). The incidence of ILD is low, with less than 1% occurring in U.S. clinical trials but upward of 4% in the Japanese population.31 The mechanism for this adverse effect is not clearly understood, but it is thought that EGFR plays an important role in repairing lung damage. Inhibiting this pathway would allow patients to be more susceptible to acute lung injuries. Patients should be told that this is a life-threatening event, and any acute sign of shortness of breath and cough with fever should require immediate medical attention. Therapy with the TKI should be held until ILD can be ruled out.31

Neutropenia: Neutropenia is a concern only with imatinib, dasatinib, and nilotinib, the TKIs that target the BCR-ABL fusion protein found in leukemia. This is most likely due to the nature of the disease, rather than to the drug's direct cytotoxic effect on neutrophil progenitor cells. For example, clinical trials observed a higher frequency of neutropenia in patients with more advanced leukemia. Additionally, Phase I studies with imatinib observed a dose-related relationship with neutropenia.21 Therefore, patients should be told that their physician will be monitoring their white blood cell count and will adjust their dose if necessary.

Hepatic Toxicity: Hepatotoxicity is also a concern specific to the BCR-ABL inhibitors. Clinical trials with imatinib, dasatinib, and nilotinib reported elevations in bilirubin and other liver function tests associated with the drugs.21-23 Therefore, it is important to monitor the patient's liver function at routine intervals (e.g., monthly) while these medications are being taken. Furthermore, the concomitant use of acetaminophen is generally not recommended because of the increased potential for liver toxicity. In fact, there was one case of acute liver failure in the setting of concomitant acetaminophen and imatinib use that resulted in the patient's death.21

Drug Interactions
All currently available TKIs are substrates of CYP3A4. Additionally, imatinib inhibits CYP2D6 and nilotinib inhibits 2C8, 2C9, and 2D6. Dose adjustments to the TKIs are necessary when coadministering with potent CYP inhibitors or inhibitors, such as azole antifungals or rifampin. Specific dose adjustments for each drug can be found in the prescribing information.18-24

Counseling Points
When counseling individuals regarding these medications, it is important that patients have realistic expectations of therapy. Most TKIs do not cure the disease. Rather, they prolong disease progression and improve survival. Conversely, imatinib has displayed high long-term remission rates in CML, effectively "curing" most patients as long as they remain on the drug. Data are too preliminary to make the same conclusions with dasatinib and nilotinib.29 Pharmacists must proceed with caution when discussing expectations of therapy, since it is unknown what was already discussed with the oncologist.

Since a major adverse effect with the EGFR inhibitors is an acnelike rash, patients should be told to expect this reaction and contact their physician if it occurs. Recommending an OTC acne medication without a physician's evaluation is not appropriate. However, the presence of the rash can be reassuring, since clinical studies have demonstrated a correlation between this rash and response to therapy.30

A very common question from patients to pharmacist about any medication is how to take the drug with regard to meals. With TKIs, there is no consistency in this area. Therefore, each TKI will have its own recommendation about taking the drug with or without food. Even more confusing is the rationale for such recommendations; sometimes they are counterintuitive. For example, lapatinib's recommendation is to take on an empty stomach (1 hour before or after a meal), but the pharmacokinetic profile shows increased absorption with a fatty meal (area under the curve [AUC] about four times higher with food than without).18 One would think the patient should take the drug with food to maximize exposure and hopefully boost efficacy. However, it should be noted that the clinical trials specified taking the drug on an empty stomach. As such, we are unsure about what additional toxicities patients may experience because of the increased systemic exposure from taking the drug with food. Therefore, it is best to recommend taking each drug exactly as the prescribing information indicates (TABLE 2).18-24


Summary

Small-molecule TKIs represent one of the newest additions to the already vast array of therapeutic agents available to treat cancer. While tyrosine kinase inhibition is clearly a viable pharmacologic strategy for the treatment of many types of cancer, it is not a widespread cure for the disease. There appears to be a role for protein tyrosine kinases in the development of many, but not all, types of cancers. The effectiveness of these therapies depends largely on the specific pathology of the cancer in question. Their oral availability, selective nature, and relative lack of systemic toxicities make these drugs an exciting new treatment paradigm. It is apparent that the community pharmacist, who in the past had little or no role in the treatment of cancer, can and will play a more important role by dispensing and counseling patients about these agents.


REFERENCES

1. Pawson T. Regulation and targets of receptor tyrosine kinases. Eur J Cancer. 2002;38(suppl 5):S3-S10.
2. Hynes NE, Lane HA. ERBB receptors and cancer: the complexity of targeted inhibitors. Nat Rev Cancer. 2005;5:341-354.
3. Olayioye MA, Neve RM, et al. The ErbB signaling network: receptor heterodimerization in development and cancer. EMBO J.2001;19:3159-3167.
4. Ferrara N, Gerber HP, LeCouter J. The biology of VEGF and its receptors. Nat Med. 2003;9:669-676.
5. Parikh AA, Ellis LM. The vascular endothelial growth factor family and its receptors. Hematol Oncol Clin North Am.2004;18:951-971.
6. Gilliland DG, Griffin JD. The roles of FLT3 in hematopoiesis and leukemia. Blood. 2002;100:1532-1542.
7. Chang YM, Kung HJ, Evans CP. Nonreceptor tyrosine kinases in prostate cancer. Neoplasia. 2007;9:90-100.
8. Krause DS, Van Etten RA. Tyrosine kinases as targets for cancer therapy. N Engl J Med. 2005;353:172-187.
9. Lynch T, Bell DW, et al. Activating mutations in the epidermal growth factor receptor underlying responsiveness of non-small cell lung cancer to gefitinib. N Engl J Med. 2004;350:2129-2139.
10. Van Etten RA. Cycling, stressed-out and nervous: cellular functions of c-Abl. Trends Cell Biol. 1999;9:179-186.
11. Kurzrock R, Kantarjian HM, et al. Philadelphia chromosome-positive leukemias: from basic mechanisms to molecular therapeutics. Ann Intern Med. 2003;138:819-830.
12. Faderl S, Talaz M, et al. The biology of chronic myeloid leukemia. N Engl J Med. 1999;341:164-172.
13. Shawver LK, Slamon D, Ullrich A. Smart drugs: tyrosine kinase inhibitors in cancer therapy. Cancer Cell. 2002;1:117-123.
14. Arora A, Scholar EM. Role of tyrosine kinase inhibitors in cancer therapy. J Pharmacol Exp Ther. 2005;315:971-979.
15. Weinstein IB. Addiction to oncogenes–the Achilles heal of cancer. Science. 2002;297:63-64.
16. Hanahan D, Weinberg RA. The hallmarks of cancer. Cell. 2000;100:57-70.
17. Camp ER, Summy J, et al. Molecular mechanisms of resistance to therapies targeting the epidermal growth factor receptor. Clin Cancer Res. 2005;11:397-405.
18. Tykerb (lapatinib) package insert. Research Triangle Park, NC: GlaxoSmithKline; July 2008.
19. Tarceva (erlotinib) package insert. Melville, NY: OSI Pharmaceuticals, Inc; May 2007.
20. Iressa (gefitinib) package insert. Wilmington, DE: AstraZeneca; June 2005.
21. Gleevec (imatinib) package insert. East Hanover, NJ: Novartis; November 2007.
22. Sprycel (dasatinib) package insert. Princeton, NJ: Bristol-Myers Squibb; June 2006.
23. Tasigna (nilotinib) package insert. East Hanover, NJ: Novartis; October 2007.
24. Sutent (sunitinib) package insert. New York, NY: Pfizer; May 2008.
25. Tyrosine kinase inhibitors. www.clinical trials.gov. Accessed September 18, 2008.
26. Geyer CE, Forster J, Lindquist D. Lapatinib plus capecitabine for HER2-positive advanced breast cancer. N Engl J Med. 2006;355:2733-2743.
27. Shepherd FA, Rodrigues PJ, et al. Erlotinib in previously treated non-small cell lung cancer. N Engl J Med. 2005;353:123-132.
28. O'Brien SG, Guilhot F, et al. Imatinib compared with interferon and low-dose cytarabine for newly diagnosed chronic-phase chronic myeloid leukemia. N Engl J Med. 2003;348:994-1004.
29. Druker BJ, Guilhot F, et al. Five-year follow-up of patients receiving imatinib for chronic myeloid leukemia. N Engl J Med. 2006;355:2408-2417.
30. Segaert S, Van Cutsem E. Clinical signs, pathophysiology and management of skin toxicity during therapy with epidermal growth factor receptor inhibitors. Ann Oncol. 2005;16:1425-1433.
31. Ando M, Okamoto I, et al. Predictive factors for interstitial lung disease, antitumor response, and survival in non-small cell lung cancer patients treated with gefitinib. J Clin Oncol. 2006;24:2549-2556.

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