VALIDACION DE UN INSTRUMENTO: ESTUDIO METODOLÓGICO

HER2 is overexpressed in 15-30% of human breast cancers and is a marker for aggressive disease169–171. This has motivated the development of several therapies that target HER2, of which there are currently three different classes. Trastuzumab and trastuzumab emtansine (T-DM1) bind to the extracellular domain 4 of HER2, pertuzumab binds to the extracellular domain 2 of HER2, and lapatinib directly inhibits the kinase activity of both HER2 and EGFR465.

72 1.10.2.1 Trastuzumab and T-DM1

Trastuzumab, also known as Herceptin, was approved in 1998 for the treatment of HER2 overexpressing breast cancer in both early and advanced disease. For early breast cancer, trastuzumab can be used in the adjuvant setting as combination therapy or as a single agent after completion of a multi-modality anthracycline-based therapy regimen466. For metastatic breast cancer, trastuzumab is used either in combination with paclitaxel or as a single agent in patients whom have received at least one prior chemotherapeutic regimen466_{. Due to its extended clinical} use for the past twenty years, trastuzumab has become the standard of care for patients with HER2 positive breast cancer467,468. In 2013, trastuzumab conjugated with emtansine (T-DM1), a microtubule inhibitor, was approved for use in metastatic breast cancer patients previously treated with trastuzumab and a taxane469.

Trastuzumab and T-DM1 work by similar mechanisms of action. First, when trastuzumab binds to the extracellular domain 4 of HER2, it inhibits formation of ligand-independent HER2- HER3 heterodimers, which form when HER2 is overexpressed470. Second, trastuzumab binding prevents the proteolytic cleavage and shedding of the HER2 extracellular domain, inhibiting formation of the kinase active membrane bound fragment p95471. Third, when trastuzumab binds to HER2, it induces antibody-dependent cellular cytotoxicity (ADCC) by engaging Fc receptors on effector immune cells472. Finally, it has been proposed that trastuzumab may induce HER2 internalization and degradation through activation of c-Cbl473. In addition to these mechanisms, T- DM1 possesses cytotoxic mechanisms of action due to the conjugated emtansine. Internalization of HER2 – T-DM1 complexes allows lysosomal degradation of MCC, a non-reducible thioether that links trastuzumab and emtansine, which releases emtansine into the cell474_{. Subsequently,} emtansine binds to microtubules and inhibits their assembly, resulting in mitotic arrest or

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catastrophe, and eventual cell death474. For additional information on the mechanisms of action of other anti-microtubule agents, see Section 1.9.3, “Mitotic Inhibitors”.

Resistance to trastuzumab arises through multiple mechanisms. First, polymorphisms in the Fc receptor on immune cells may decrease trastuzumab-mediated ADCC and result in worse prognosis475. Second, overexpression of other HER family members (in particular EGFR and HER3) and other tyrosine kinase receptors, such as insulin growth factor 1 receptor (IGF1R), are commonly implicated in trastuzumab resistance476_{. Third, alterations of HER2, including shedding} of the extracellular domain or expression of splice variants, can decrease response to trastuzumab477. Finally, hyper-activation of the PI3K – Akt signaling pathway by multiple mechanisms has been shown to cause trastuzumab resistance476. The mechanisms of resistance to T-DM1 are not well understood but are likely associated with decreased HER2 expression, shedding of the extracellular domain, altered receptor-complex internalization, or drug efflux by MDR1474.

Both trastuzumab and T-DM1 are administered by intravenous infusion. While trastuzumab has a relatively long elimination half-life of 28 days, T-DM1 has a shorter half-life of 4 days due to proteolytic degradation of the emtansine linker466,478,479. The elimination pathways of trastuzumab and T-DM1 are unknown but are hypothesized to be similar to homeostatic management of immunoglobulin G466. The most clinically significant toxicities associated with trastuzumab treatment are cardiotoxicity and pulmonary toxicity, which manifest most aggressively as congestive heart failure and pneumonitis respectively466,480. The most clinically significant toxicities associated with T-DM1 use are thrombocytopenia and transaminitis (the elevation of AST and ALT), without any reports of hepatic injury481_.

74 1.10.2.2 Pertuzumab

While trastuzumab is the standard of care for women with HER2 positive breast cancer, approximately 15% of women relapse despite trastuzumab therapy482. Pertuzumab was approved in 2012 for women with HER2 positive breast cancer in combination with trastuzumab and chemotherapy483,484. Pertuzumab binds to the extracellular domain 2 of HER2 and is effective at reducing ligand-dependent formation of HER2 heterodimers, most specifically heregulin-induced HER2-HER3 dimers485,486_{. In contrast, trastuzumab is only effective at blocking ligand-} independent dimerization470. Similar to trastuzumab, the Fc domain of pertuzumab can interact with the Fc receptor on immune cells and induce ADCC487. Resistance mechanisms to pertuzumab are not as well characterized as those of trastuzumab. Some proposed resistance mechanisms include formation of HER3-EGFR heterodimers, HER2 extracellular domain shedding, and PIK3CA expression488,489_.

Pertuzumab is administered by intravenous injection and has a terminal elimination half- life of approximately 19 days490. Similar to trastuzumab, the elimination of pertuzumab is thought to be conducted similarly to homeostatic management of immunoglobulin G490. The most clinically severe toxicities observed are cardiotoxicity, most commonly a decrease in left ventricular ejection fraction (LVEF). Importantly, LVEF dysfunction incidence is not significantly increased when pertuzumab is added to trastuzumab therapy491.

1.10.2.3 Lapatinib

Lapatinib is a dual specificity EGFR/HER2 small molecule kinase inhibitor. It is currently approved for women with HER2 positive breast cancer in combination with capecitabine who have received prior therapy with trastuzumab, an anthracycline, and a taxane492_{. Additionally, lapatinib,} in combination with letrozole, is approved for use in hormone receptor positive metastatic breast

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cancer that overexpresses HER2492. The mechanism of action of lapatinib is inhibition of EGFR and HER2 kinase activity by acting as an analogue of ATP to prevent ATP-mediated transphosphorylation of receptor dimers493. Additionally, lapatinib can also impact HER3 phosphorylation in the context of EGFR-HER3 or HER2-HER3 heterodimers494. Disruption of these receptor tyrosine kinases inhibits downstream signaling through MAPK, PI3K-Akt, and PLCγ494_{. Resistance to lapatinib most commonly occurs with activation of compensatory} pathways, including other receptor tyrosine kinases and intracellular kinases, or mutation of the tyrosine kinase domain of HER2495.

Lapatinib is an orally administered agent and exhibits an elimination half-life of 24 hours493. Lapatinib is primarily eliminated by biliary excretion after hepatic metabolism by CYP3A4496. Importantly, lapatinib metabolites can act as covalent inhibitors of CYP3A4, which can cause drug-drug interactions and hepatotoxicity497,498_{. Lapatinib is generally well-tolerated,} with the most common adverse effects being diarrhea, rash, nausea, and fatigue499.