Drug-induced thrombocytopenia is an adverse reaction characterized by accelerated platelet destruction. The present study described a case of thrombocytopenia that occurred during treatment with panitumumab. A female patient aged 49 years with metastatic rectal adenocarcinoma was treated with 9 out of 12 cycles of therapy with the standard of care, 5-fluorouacil (5-FU), oxaliplatin and folic acid, in association with panitumumab. During cycle 10, the patient developed severe thrombocytopenia, so the therapy was adjusted to a lower dosage; however, during cycle 11, after administration of panitumumab and before administration of 5-FU or oxaliplatin, the patient again presented with severe thrombocytopenia, with a platelet count <2×109/l. Immunology test results were negative apart from anti-nucleus antibodies (titration, 1:160). Naranjo's algorithm was used to establish the relationship between the use of panitumumab and thrombocytopenia onset and a score of 6 ('probable') was found. The temporal link between the onset of symptoms and administration of therapy, the relapse of thrombocytopenia after re-administration of the drug during cycle 11 (positive rechallenge) and Naranjo score of 6 ('probable') are crucial elements for establishing the causal relationship and the probability that thrombocytopenia was related to the administration of panitumumab. The patient then underwent two cycles of therapy with 5-FU, folic acid and irinotecan, in association with bevacizumab, experiencing again the same adverse event. Treatment with monoclonal antibodies was suspended altogether in favor of a switch to trifluridine/tipiracil. No other serious adverse events were reported.
Keywords: adverse drug reaction; bevacizumab; colorectal cancer; panitumumab; thrombocytopenia.
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