PD-1 and PD-L1 inhibitors
PD-1 inhibitors and PD-L1 inhibitors are a novel group of checkpoint inhibitors being developed for the treatment of cancer. PD-1 and PD-L1 are both proteins present on the surface of cells. Immune checkpoint inhibitors such as these are emerging as a front-line treatment for several types of cancer.
PD-1 and PD-L1 inhibitors act to inhibit the association of the programmed death-ligand 1 (PD-L1) with its receptor, programmed cell death protein 1 (PD-1). The interaction of these cell surface proteins is involved in the suppression of the immune system and occurs following infection to limit the killing of bystander host cells and prevent autoimmune disease. This immune checkpoint is also active in pregnancy, following tissue allografts, and in different types of cancer.
The concept of blocking PD-1 and PD-L1 for the treatment of cancer was first published in 2001. Pharmaceutical companies began attempting to develop drugs to block these molecules, and the first clinical trial was launched in 2006, evaluating nivolumab. As of 2017, more than 500 clinical trials involving PD-1 and PD-L1 inhibitors have been conducted in more than 20,000 patients. By the end of 2017, PD-1/PD-L1 inhibitors had been approved for the treatment of nine forms of cancer.
In the cancer disease state, the interaction of PD-L1 on the tumor cells with PD-1 on a T-cell reduces T-cell function signals to prevent the immune system from attacking the tumor cells. Use of an inhibitor that blocks the interaction of PD-L1 with the PD-1 receptor can prevent the cancer from evading the immune system in this way. Several PD-1 and PD-L1 inhibitors are being trialled within the clinic for use in advanced melanoma, non-small cell lung cancer, renal cell carcinoma, bladder cancer and Hodgkin lymphoma, amongst other cancer types.
Immunotherapy with these immune checkpoint inhibitors appears to shrink tumours in a higher number of patients across a wider range of tumour types and is associated with lower toxicity levels than other immunotherapies, with durable responses. However, de-novo and acquired resistance is still seen in a large proportion of patients. Hence PD-L1 inhibitors are considered to be the most promising drug category for many different cancers.
PD-1 and PD-L1 inhibitors are closely related to CTLA4 (cytotoxic T-lymphocyte-associated protein 4) inhibitors, such as ipilimumab. PD-1 and CTLA-4 are both expressed on activated T cells, but at different phases of immune response.
Pembrolizumab (formerly MK-3475 or lambrolizumab, Keytruda) was developed by Merck and first approved by the Food and Drug Administration in 2014 for the treatment of melanoma. It was later approved for metastatic non-small cell lung cancer and head and neck squamous cell carcinoma. In 2017, it became the first immunotherapy drug approved for use based on the genetic mutations of the tumor rather than the site of the tumor. It was shown, that patients with higher non-synonymous mutation burden in their tumors respond better to the treatment. Both their objective response rate and progression-free survival was shown to be higher than in patients with low non-synonymous mutation burden. This suggests that tobacco smoking, due to its high carcinogenicity, not only causes promotion of cancer (in case of non-small-cell lung carcinoma), but also increases chances of the immune system to recognize and attack to the tumor.
Nivolumab (Opdivo) was developed by Bristol-Myers Squibb and first approved by the FDA in 2014 for the treatment of melanoma. It was later approved for squamous cell lung cancer, renal cell carcinoma, and Hodgkin's lymphoma.
As of 2017, at least five PD-1 inhibitors were under development.
- pidilizumab, by Cure Tech
- AMP-224, by GlaxoSmithKline
- AMP-514, by GlaxoSmithKline
- PDR001, by Novartis
- cemiplimab, by Regeneron and Sanofi
Atezolizumab (Tecentriq) is a fully humanised IgG1 (immunoglobulin 1 antibody developed by Roche Genentech. In 2016, the FDA approved atezolizumab for urothelial carcinoma and non-small cell lung cancer.
Avelumab (Bavencio) is a fully human IgG1 antibody developed by Merck Serono and Pfizer. Avelumab is FDA approved for the treatment of metastatic merkel-cell carcinoma. It failed phase III clinical trials for gastric cancer.
Durvalumab (Imfinzi) is a fully human IgG1 antibody developed by AstraZeneca. Durvalumab is FDA approved for the treatment of urothelial carcinoma and unresectable non-small cell lung cancer after chemoradiation.
At least two PD-L1 inhibitors are in the experimental phase of development.
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Immunotherapies as a group have off-target effects and toxicities common to them. Some of these include interstitial pneumonitis, colitis, skin reactions, immune thrombocytopenia, neutropenia, encephalopathy, Guillain-Barré syndrome, myelitis, myasthenia gravis, myocarditis and cardiac insufficiency, acute adrenal insufficiency, and nephritis.
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