Vectibix (panitumumab) is a targeted cancer medicine used to treat RAS wild-type metastatic colorectal cancer (mCRC). It is a monoclonal antibody that targets the epidermal growth factor receptor (EGFR), a protein on the surface of cancer cells that drives their growth.

By binding to EGFR, panitumumab blocks the signal that tells cancer cells to grow and divide. It is used either alone or in combination with chemotherapy regimens.

Testing for RAS mutations (KRAS and NRAS genes) is mandatory before prescribing Vectibix. It is only effective in patients whose tumour has no mutations in the RAS genes (RAS wild-type). In patients with RAS mutations, Vectibix provides no benefit and is not prescribed.

Vectibix is given as an intravenous (IV) infusion over 60 minutes (first dose) or 30 minutes (if well tolerated) every two weeks. It is administered in a hospital or cancer centre by trained medical staff.

Pre-medication (antihistamines) is recommended to reduce the risk of infusion reactions.

Your infusions are scheduled by your oncology team. Contact them if you need to reschedule an appointment.

In a clinical setting, any dosing error is managed immediately by medical staff.

Storage is managed by the treating clinic. Vials are stored in a refrigerator between 2°C and 8°C.

Side effects are common and some are unique to EGFR-targeting medicines.

Very common side effects:

• Skin toxicity: the most common and often significant side effect. Includes acne-like rash, dry skin, skin fissures, and nail changes. This is similar to the rash seen with gefitinib and often correlates with treatment response
• Hypomagnesaemia (low magnesium): EGFR inhibition in the kidneys causes loss of magnesium. Magnesium levels are checked regularly and supplements prescribed when needed
• Fatigue
• Nausea and diarrhoea

Serious side effects:

• Interstitial lung disease or pneumonitis: new cough or breathlessness should be reported promptly
• Infusion reactions: premedication is given but reactions can still occur
• Severe skin reactions in rare cases
• Eye toxicity (keratitis): if you develop eye pain or irritation, contact your oncologist

Blood tests including magnesium, liver function, and blood count are monitored during treatment.

There are no significant pharmacokinetic drug-drug interactions with panitumumab as a monoclonal antibody. However, it is often given with chemotherapy, so the interactions of those medicines apply.

Tell your oncologist about all medicines, supplements, and herbal products.

Why is RAS testing so important before Vectibix?

RAS mutations (particularly in KRAS and NRAS genes) make colorectal cancer cells unresponsive to EGFR inhibition. In patients with RAS-mutant tumours, Vectibix not only provides no benefit but may cause harm. Multiple large clinical trials have confirmed that EGFR antibodies like panitumumab and cetuximab are only effective in RAS wild-type (unmutated) colorectal cancer. Testing is therefore not optional.

What can I do about the skin rash?

Skin rash is very common with Vectibix. Paradoxically, a skin rash often indicates the medicine is working and correlates with better outcomes. Keeping skin moisturised, avoiding sun exposure on affected areas, using gentle skincare products, and wearing sun protection can help manage the rash. Your oncologist may prescribe topical or oral antibiotics or steroids if the rash is severe.

Ace provides accurate and independent information medically reviewed on prescription medications. This material is provided for educational purposes only and is not intended for medical advice, diagnosis or treatment.