[30][33] Common side effects include fever, infection, cough, headache, trouble sleeping, and rash.

[35] The safety and efficacy of trastuzumab-containing combination therapies (with chemotherapy, hormone blockers, or lapatinib) for the treatment of metastatic breast cancer.

[38] The original studies of trastuzumab showed that it improved overall survival in late-stage (metastatic) HER2-positive breast cancer from 20.3 to 25.1 months.

[42] It is surprising that although trastuzumab has great affinity for HER2 and high doses can be administered (because of its low toxicity), 70% of HER2+ patients do not respond to treatment.

A mechanism of resistance involves failure to downregulate p27 (Kip1) [43] as well as suppressing p27 translocation to the nucleus in breast cancer, enabling cdk2 to induce cell proliferation.

[44] In May 2021, the FDA approved pembrolizumab in combination with trastuzumab, fluoropyrimidine- and platinum-containing chemotherapy for the first-line treatment of people with locally advanced unresectable or metastatic HER2 positive gastric or gastroesophageal junction (GEJ) adenocarcinoma.

[45] The optimal duration of add-on trastuzumab treatment after surgery for early breast cancer is unknown.

Debate about treatment duration has become a relevant issue for many public health policy makers because administering trastuzumab for a year is very expensive.

Consequently, some countries with a taxpayer-funded public health system, such as New Zealand, chose to fund limited adjuvant therapy.

[50] Some of the common side effects of trastuzumab are flu-like symptoms (such as fever, chills and mild pain), nausea and diarrhea.

As a result, regular cardiac screening with either a MUGA scan or echocardiography is commonly undertaken during the trastuzumab treatment period.

[53] Trastuzumab downregulates neuregulin-1 (NRG-1), which is essential for the activation of cell survival pathways in cardiomyocytes and the maintenance of cardiac function.

In some types of cancer, the pathway is exploited to promote rapid cell growth and proliferation and hence tumor formation.

Signaling compounds called mitogens (specifically EGF in this case) arrive at the cell membrane, and bind to the extracellular domain of the HER family of receptors.

Signals on these pathways promote cell proliferation and the growth of blood vessels to nourish the tumor (angiogenesis).

It is thought that a contribution to the unregulated growth observed in cancer could be due to proteolytic cleavage of HER2/neu that results in the release of the extracellular domain.

In the routine clinical laboratory, the most commonly employed methods for this are immunohistochemistry (IHC) and either silver, chromogenic or fluorescent in situ hybridisation (SISH/CISH/FISH).

Virtual karyotyping has the added advantage of assessing copy number changes throughout the genome, in addition to detecting HER-2 amplification (but not overexpression).

The main expense involved with CISH is in the purchase of FDA-approved kits, and as it is not a fluorescent technique it does not require specialist microscopy and slides may be kept permanently.

As of June 2011 Roche has obtained FDA approval for the INFORM HER2 Dual ISH DNA Probe cocktail [70] developed by Ventana Medical Systems.

[72] The drug was first discovered by scientists including Axel Ullrich and H. Michael Shepard at Genentech, Inc. in South San Francisco, CA.

A book about Dr. Slamon's work was made into a television film called Living Proof, that premiered in 2008. Genentech developed trastuzumab jointly with UCLA, beginning the first clinical trial with 15 women in 1992.

[80] Since October 2006, trastuzumab has been made available for Australian women and men with early-stage breast cancer via the Pharmaceutical Benefits Scheme.

[83] On 16 September 2014, Genentech notified hospitals in the United States that, as of October, trastuzumab could only be purchased through their selected specialty drugs distributors not through the usual general line wholesalers.

[84] By 2014, around 20 companies, particularly from emerging markets, were developing biosimilar versions of trastuzumab after Roche/Genentech's patents expired in 2014 in Europe, and in 2019 in the United States.

[85] In January 2015, BIOCAD[clarification needed] announced the first trastuzumab biosimilar approved by the Ministry of Health of the Russian Federation.

Iran also approved its own version of the monoclonal antibody in January 2016, as AryoTrust, and announced its readiness to export the drug to other countries in the Middle-East and Central Asia when trade sanctions were lifted.

[88] Trastuzumab-dkst (Ogivri, Mylan GmbH) was approved in the United States in December 2017, to "treat people with breast cancer or gastric or gastroesophageal junction adenocarcinoma whose tumors overexpress the HER-2 gene.

[5] In July 2024, the CHMP adopted a positive opinion, recommending the granting of a marketing authorization for the medicinal product Tuznue, intended for the treatment of breast and gastric cancer.