Cancer inhibitors, also known as “cancer growth blockers” are an emerging and promising targeted cancer drug. Cancer inhibitors work by literally blocking the growth factors that allow cancerous cells to divide and grow. The aim of treating cancer with a targeted cancer inhibitor is to block the signalling process that trigger this division and growth of cancer cells by doing the following:

• Preventing the growth factor receptor on cancerous cells
• Decreasing growth factor levels throughout the patient’s body
• Inhibiting signals inside cancerous cells that trigger (i.e., blood vessels) growth

Researchers are working to develop specific inhibitors to treat various cancer growth factors. The following targeted cancer therapies often overlap, meaning they may be used to treat more than one type of cancer:

1. Aromatase inhibitors

According to Breastcancer.org, aromatase inhibitors aim to halt the production of estrogen in postmenopausal women by blocking the enzyme aromatase, which converts androgen hormone  into estrogen. By decreasing estrogen levels, less hormone is able to trigger the growth of hormone-receptor-positive breast cancer cells. Aromatase inhibitors for breast cancer treatment are available in Femara (or exemestane), Arimidex (or anastrozole), and Aromasin (or exemestane) oral pills taken daily.

2. Histone deacetylase inhibitors

Histone deacetylase inhibitors (also known as HDACi and HDIs) are chemical compounds that inhibit histone deacetylase. They’ve long been used to treat mood disorders and epileptic patients, but in recent times have been considered as promising in the suppression of cancerous cell growth and can be used as chemotherapeutic agents under the names Zolinza (suberoylanilide hydroxamic acid or SAHA) used to treat cutaneous T‐cell lymphoma, Belinostat for treating T‐cell lymphoma, and Panobinstat for treating multiple myeloma patients.

3. PARP inhibitors

PARP inhibitors block the enzyme poly ADP ribose polymerase (otherwise known as PARP) in certain cancers that depend on PARP for cell growth. While PARP inhibitors are often used to treat several neurological disorders, stroke, and myocardial infarction, they also show promise in the treatment of ovarian cancer, specifically in patients with recurrent platinum-sensitive ovarian cancer. Approved PARP inhibitors include Rucaparib (or Rubraca) and Olaparib (or Lynparza) both used in the treatment of advanced  BRCA mutant ovarian cancer, and Niraparib (or Zejula) used to treat primary peritoneal, epithelial ovarian, and fallopian tube cancers.

4. FLT3 inhibitors

FLT3 inhibitors are used in the treatment of acute myeloid leukemias (AMLs), which is amongst the most common leukemias in adults. Sadly, if AML is not treated early enough, the cancer can quickly become fatal. Mutations of the receptor, tyrosine kinase is necessary for AML cancerous cell growth. However, FLT3 inhibitors (i.e., Sunitinib, Midostaurin, and Lestaurtinib) show promise in clinical trials for clinical anti-leukemia effects and remissions.

5. Btk inhibitors

Bruton’s tyrosine kinase (or Btk) inhibitors block B cell antigen receptors from binding to pathogens (i.e., bacteria, viruses, and cancerous cells) and triggers abnormal cell cycle growth  (or development of B-cell lymphoma), and limits the survival of these abnormal cells. BTK inhibitors, such as Calquence (acalabrutinib) and Ibrutinib (Imbruvica) block the BTK protein in the early treatment of several types of Non-Hodgkin Lymphoma (i.e., small lymphocytic lymphoma, mantle cell lymphoma, and marginal zone lymphoma), taken in oral capsule form.

6. PI3K inhibitors

PI3K is a protein linked to the development of several lymphomas (i.e., namely follicular lymphoma and small lymphocytic lymphoma) after other cancer treatments have failed. P13K inhibitors are marketed as Copanlisib (or Aliqopa), given as an injection to treat follicular lymphoma, and Idelalisib (or Zydelig) taken as in pill form to treat follicular lymphoma and small lymphocytic lymphoma.

7. Proteasome inhibitors

Commonly used to treat multiple myeloma and some non-Hodgkin lymphomas, Proteasome inhibitors block proteasomes (or enzyme complexes) within cells from breaking down and causing abnormal cell division, and resulting in cancerous cell growth. The Proteasome inhibitor
Bortezomib (or Velcade) is given via injection or infusion.

8. CDK inhibitors

Cyclin-dependent kinase (or CDK) inhibitors aim to prevent the function of CDK proteins, which trigger the production of cancer cells in postmenopausal women with estrogen receptor positive and HER2 negative forms of breast cancer. The CDK4 and CDK6 inhibitors (marketed as Ribociclib, Abemaciclib, and Palbociclib) are often administered to advanced HER2-negative metastatic breast cancer patients in combination with letrozole to slow the disease progression and improve overall breast cancer survival rates. While almost a dozen CDK inhibitors are currently subject to clinical trials in the treatment of melanomas and leukemias.

9. mTOR inhibitors

The mTOR protein (also known as the mechanistic target of rapamycin) is responsible for the development of uncontrolled tumor cell growth that causes a specific form of pancreatic cancer, known as pNET. Sadly, roughly two thirds of pNET diagnosis are made after the cancer has spread to other tissues or organs in the body, drastically reducing life expectancy to a maximum of 27 months. The mTOR inhibitor Sunitinib (when combined with Everolimus) is FDA-approved in the treatment of pNET. However, the mTOR inhibitor, Everolimus, has also shown promise in the treatment of some breast cancers, kidney (renal) cancers, and cell carcinomas.

10. Hedgehog pathway inhibitors

Roughly 2.8 million new basal cell carcinoma (BCC) diagnosis are made each year in the U.S., resulting in approximately 3000 deaths annually. However, hedgehog pathway inhibitors are used to treat cancers that develop due to hedgehog (Hh) signaling pathways. Hh inhibitors are FDA-approved under names such as Vismodegib and Sonidegib (as well as the antifungal drug Itraconazole) to treat basal cell carcinoma, often with with aggressively large and disfiguring metastatic tumors.