Imatinib

Mechanism of Action

Imatinib mesylate is a 2-phenylamino-pyrimidine derivative protein and tyrosine kinase inhibitor initially targeted to the platelet-derived growth factor receptor.[9] Subsequently, it has also been found to inhibit other protein tyrosine kinases such as c-kit (gastrointestinal stromal tumors) and BCR-ABL fusion protein (Philadelphia chromosome chronic myelogenous leukemia). These protein tyrosine kinases as a whole phosphorylate specific amino acids on substrate proteins, which induce signal transduction resulting in changes to cell biology, including cell growth, differentiation, and death, constitutive activation of which, through mutation or other means, can lead to malignancy.[10]

Blocking this constitutive action has been shown to induce downstream apoptosis without further differentiation.[11] Imatinib, as a therapeutic, blocks the constitutive action of protein tyrosine kinase by working as a competitive inhibitor of the ATP binding cleft of ABL, inducing apoptosis of leukemic cells.[12]

Administration

Imatinib is FDA-approved to be administered orally and is typically supplied in 100 mg and 400 mg tablet formulations. Imatinib was initially available under a branded name; however, after the expiration of several patents, generic versions are available. The recommended dosing for imatinib varies by indication:

• Adult Ph+ CML CP 400 mg/day
• Adult Ph+ CML AP 600 mg/day
• Adult Ph+ CML BC 600 mg/day
• Pediatric Ph+ CML 340 mg/m2/day
• Adult Ph+ ALL 600 mg/day
• Adult MDS 400 mg/day
• Adult MPD 400 mg/day
• Adult ASM 100 mg/day or 400mg/day
• Adult HES 100 mg/day or 400mg/day
• Adult CEL 100 mg/day or 400mg/day
• Adult DFSP 800 mg/day
• Adult metastatic/unresectable GIST 400 mg/day
• Adult adjuvant GIST 400 mg/day
• Mild to moderate hepatic impairment 400 mg/day
• Severe hepatic impairment 300 mg/day

800 mg doses should be administered as a 400 mg dose twice daily; all other doses should be given once daily. Patients should take imatinib with meals and a large glass of water to minimize gastrointestinal upset, and food does not impact bioavailability.[13]

Adults of Reproductive Potential

Human post-marketing surveillance and animal studies have shown imatinib to harm the developing fetus. Therefore, clinicians should test pregnancy status in females with reproductive potential before the initiation of treatment.

Contraception

Advise females of reproductive potential to administer effective contraception (methods resulting in less than 1% pregnancy rates) while using imatinib during treatment and for two weeks days after stopping treatment with imatinib.

Pregnancy Considerations

Based on human and animal data, imatinib can cause fetal harm when administered to a pregnant woman. There are no clinical studies on the use of imatinib in pregnant women. However, there have been postmarket reports of spontaneous abortions and congenital anomalies from women who have been exposed to imatinib during pregnancy. Therefore, advise females to avoid pregnancy when taking imatinib. If imatinib is administered during pregnancy, or if the female gets pregnant while taking it, counsel the patient about the potential hazard for the fetus.[14][15]

Breastfeeding Considerations

Imatinib and its metabolite are excreted into breast milk. Since there is potential for serious adverse reactions in breastfed infants from imatinib, advise a lactating woman not to breastfeed during treatment and for one month after the last dose.[16][17]

Hepatic Impairment

Mild and moderate hepatic impairment do not affect exposure to imatinib and metabolite CGP74588. However, in patients with severe hepatic impairment, the imatinib AUC increased by 45%, and the CGP74588 AUC increased by 55%, compared to patients with normal hepatic function. Therefore, reduce the dose of imatinib by 25% for patients with severe hepatic impairment.

Renal Impairment

There is insufficient data on patients with severe renal impairment, and as per the manufacturer’s labeling, dose reductions are required for patients with moderate and/or severe renal impairment.

Adverse Effects

Clinically Significant Warnings and Precautions

There are several clinically relevant precautions with imatinib use.[18] These include:

• Dermatologic: including cases of Stevens-Johnson syndrome (SJS) and erythema multiforme (although the etiology of imatinib causing SJS is unclear)
• Embryo-fetal toxicity
• Fluid retention and edema
• Gastrointestinal disorders and perforations
• Growth retardation in children
• Hematologic toxicity (anemia, neutropenia, thrombocytopenia)
• Hemorrhage (gastrointestinal and intra-tumoral grade 3/4 hemorrhages)
• Hepatotoxicity
• Hypereosinophilic cardiac toxicity (cardiogenic shock and left ventricular dysfunction have been reported in HES patients initiating imatinib therapy)
• Hypothyroidism
• Long-term use toxicity (cardiac, kidney, and liver toxicities suggested by animal studies)
• Pregnancy (category D – avoid pregnancy while taking imatinib)
• Renal toxicity
• Severe congestive heart failure with left ventricular dysfunction
• Tumor lysis syndrome

Common Side Effects That Occurred in Over 10% of Clinical Trial Participants

Imatinib has a long list of potential adverse events.[8][5] The following list many of the most relevant examples: 

• Abdominal pain/distention
• Alanine aminotransferase increase
• Alopecia
• Anemia
• Anorexia
• Arthralgia
• Aspartate aminotransferase increase
• Asthenia
• Blood creatine increase
• Blood lactate dehydrogenase increase
• Bone pain
• Constipation
• Cough
• Depression
• Diarrhea
• Dizziness
• Dyspepsia
• Dyspnea
• Fatigue
• Flatulence
• Fluid retention
• Headache
• Hemoglobin decrease
• Hemorrhage (non-GI/CNS)
• Hypoalbuminemia
• Hypokalemia
• Hypoproteinemia
• Infection
• Influenza
• Insomnia
• Joint pain
• Muscle cramps/spasms
• Musculoskeletal pain
• Myalgia
• Nasopharyngitis
• Nausea
• Neutropenia
• Night sweats
• Pain
• Peripheral edema
• Periorbital edema
• Pharyngolaryngeal pain
• Pneumonia
• Pruritis
• Pyrexia
• Rhinitis
• Rigors
• Skin rash
• Sinusitis
• Thrombocytopenia
• Upper respiratory tract Infection
• Vision, blurred
• Vomiting
• Weight increase
• White blood cell decrease

Contraindications

There are no contraindications for imatinib.

Imatinib gets metabolized through the enzyme cytochrome p450 3A4 (CYP3A4).[19] 

• Administration of imatinib with CYP3A4 inducers may reduce total imatinib exposure.
• Administration of imatinib with CYP3A4 inhibitors may increase total imatinib exposure.
• Avoid grapefruit juice as it can potentially increase the plasma concentration of imatinib.
• Imatinib will increase plasma concentrations of other CYP3A4 metabolized drugs.

Monitoring

• Monitor patients regularly for the signs and symptoms of fluid retention.
• Monitor patients for signs and symptoms of cardiac or renal failure, especially in those with a history of cardiac disease or renal failure
• Monitor liver function (alkaline phosphatase, bilirubin, and transaminases) before starting treatment and then monthly afterward or as indicated going forward
• Monitor hepatic function
• Monitor TSH levels in patients after thyroidectomy[20]
• Monitor growth in children
• Monitor patients with a high proliferative rate or high tumor burden for tumor lysis syndrome
• Monitor renal function before starting therapy and on an ongoing basis, especially in those with congestive heart failure, diabetes mellitus, or pre-existing renal impairment
• Monitor complete blood counts for evidence of hematologic toxicity (anemia, neutropenia, and thrombocytopenia)

Toxicity

• Treatment with imatinib is associated with hematologic toxicity (anemia, neutropenia, and thrombocytopenia)
• Treatment with imatinib is associated with hepatotoxicity.
• Treatment with imatinib is associated with hypereosinophilic cardiac toxicity in patients with hypereosinophilic syndrome.
• Treatment with imatinib can cause fetal harm to a pregnant woman (Imatinib has demonstrated teratogenicity in rats at the 800 mg/day equivalent human maximum dose)[14]
• Treatment with imatinib is associated with renal toxicity.
• Treatment with imatinib is associated with dermatologic toxicities, including erythema multiforme and Stevens-Johnson syndrome.[18]

Enhancing Healthcare Team Outcomes

Imatinib is a powerful targeted therapeutic drug for several types of cancer. Despite showing a favorable risk/benefit tradeoff in several clinical trials and being FDA-approved for nearly two decades, it does require close monitoring for safety. In addition to the prescribing physician, pharmacists and nurses are responsible for educating the patient on the potential side effects of the drug. While most side effects occur in the first two years of use, many patients will be on chronic therapy for many years, so it is essential to stay vigilant in monitoring for side effects.[20] 

Particularly severe side effects that require careful monitoring include Stevens-Johnson syndrome, gastrointestinal perforations/hemorrhage, fluid retention, organ (cardiac, renal, and liver) failure, and hematologic toxicities. Additionally, imatinib does have several interactions with other medications that are also metabolized by CYP3A4. The pharmacist needs to monitor all medications the patient takes to ensure no interactions. This requires proper dose adjustments due to potential drug-drug interactions if concurrent medications are necessary. The clinicians, nurses, and pharmacists should regularly check liver function, renal function, and complete blood counts to monitor for organ damage or immunosuppression and adjust imatinib dosing as necessary while continually informing the ordering clinician.

Nursing will notably have the optimal chance to see any issues that arise since they will have the first contact with the patient, so they must be familiar with the adverse event and interaction profile. This is where an oncology specialty nurse is invaluable. All interprofessional team members are responsible for monitoring the patient's progress and reporting any status changes to the rest of the team so corrective measures can be taken if necessary. This also includes meticulous documentation so that everyone on the healthcare team has the same patient data set from which to work. This collaborative paradigm exemplifies the type of interprofessional teamwork the healthcare team requires to administer imatinib effectively to achieve optimal patient outcomes while minimizing patient risks. [Level 5]

It was one of the first targeted therapeutics to reach the market and also now is generic. Imatinib presents interesting time-series data on the costs of precision medications. At approval in 2001, a one-month supply of the 400mg daily dose cost $2200 in the US.[21] In 2010, this cost had ballooned to $5143 per month, and by 2018 it was $10620 per month.[22] A generic form of imatinib was launched in the US in 2015 at a 30% discount on the branded price (which was substantially higher than the 82% discount at which the Canadian generic was launched).[23]

Because of dysfunctional market maximizing forces, ironically, the price of imatinib generics was higher than the original branded version, and pricing relief remained elusive for years. However, during 2018, generic imatinib pricing finally dropped below its original launch price, yet it still cost several thousand dollars per annum despite being generic. With expensive therapies such as imatinib, the interprofessional healthcare team, including clinicians, nurses, pharmacists, and other healthcare professionals (such as social workers or charity assistance programs), must coordinate care and advocate for coverage to maximize access to life-saving medications.