Why is VEGFR Inhibition Important ?

The Vascular Endothelial Growth Factor Receptor (VEGFR) is responsible for providing the blood supply to the tumour. Inhibit VEGFR and the vascularisation to the tumour ceases and the tumour undergoes necrosis and dies off. VEGFR is also responsible for blood supply to the metastatic tumours and inhibitors of VEGFR have antimetastatic effects.

Zytiga is currently undergoing clinical trials to test the efficacy in combination with a VEGFR inhibitor.

2 clinical combinations that are in trials are:

1. Cabozantinib Zytiga Combo

2. Sunitinib Zytiga Combo

Vascular Endothelial Growth Factor Receptor (VEGFR) is the receptor of VEGF the Vascular Endothelial Growth Factor. VEGFR is involved in cell proliferation, migration, survival and permeability. The VEGFs include five known structurally-related mammalian ligands (VEGFA, VEGFB, VEGFC, VEGFD, and placenta growth factor, PLGF) and there are also three structurally related VEGFRs subtypes (VEGFR1, VEGFR2, and VEGFR3). 

Solid tumors require the growth and dissemination of blood vessels and lymphatic vessels to support the metastatic growth of cancers. Following the recognition of growth factor receptor pathways that regulate angiogenesis, a number of small molecular inhibitors and antibodies have been developed that target the activity of vascular endothelial growth factor (VEGF)-VEGF receptor (VEGFR) pathway. This includes oral small-molecule tyrosine kinase inhibitors currently in clinical practice, namely sunitinib and sorafenib. These are commonly used in the treatment of renal cell carcinoma (RCC) and hepatocellular carcinoma (HCC), two indications that are known to develop resistance to conventional chemotherapeutics.

The VEGFs include five known structurally-related mammalian ligands (VEGFA, VEGFB, VEGFC, VEGFD, and placenta growth factor, PLGF). The VEGFs are disulfide-bonded homodimers, however, VEGFA and PLGF heterodimers are also known to exist. Due to alternative splicing or due to processing, VEGF ligands occur as several different variants. As a result, these variants bind differently to both VEGFRs and to co-receptors resulting in different biological responses including angiogenesis, lymphangiogenesis, permeability, inflammatory cell recruitment and fatty acid uptake. VEGFs are produced by several different cell types and act in a paracrine manner. The VEGFs bind to three structurally related tyrosine kinases (VEGFR1, VEGFR2, and VEGFR3). Modulating the effect of the VEGFRs are a number of co-receptors that lack intrinsic catalytic activity (i.e. heparin sulfate, neurophilins and integrins) and bind to VEGF.^[1]

VEGFR1 (also known as Fms-like tyrosine kinase 1, Flt1, in mice) is a single-transmembrane glycoprotein structurally related to VEGFR2 and VEGFR3. VEGFR1 is expressed at high levels in vascular endothelial cells, and along with VEGFR2 binds to VEGFA. VEGFR1 is noted to bind exclusively to VEGFB and PIGF. Expression of VEGFR1 is noted to occur during vessel growth and remodeling activity. Non-endothelial cells that express VEGFR1 includes monocytes and macrophages, human tropholblasts, renal mesangial cells, vascular smooth muscle cells, dendritic cells and various tumor cells. A key regulator of VEGFR1 gene expression is hypoxia.^[1]

VEGFR2 (also known as KDR; kinase insert domain receptor, in the human and Flk1; fetal liver kinase-1, in mice) binds VEGFA with a 10-fold lower affinity than VEGFR1.  Other targets of VEGFR2 include proteolytically processed VEGFC and VEGFD. The only known ligand to uniquely bind to VEGFR2 is the open reading frame-encoded VEGFE. VEGFR2 is expressed in most adult vascular endothelial cells as well as circulating endothelial progenitor cells, pancreatic duct cells, retinal progenitor cells, megakaryocytes and hematopoietic cells. VEGFR2 expression is induced in conjunction with active angiogenesis (i.e. the uterus during the reproductive cycle) and in pathological process related to neovascularization (i.e. cancer). VEGFR2, often in combination with VEGFR3, is expressed at significantly upregulated levels in the tumor vascular endothelium in most common human solid tumors. Tumor cells can also express VEGFR2, however, epithelial and mesenchymal tumor cells typically express VEGFR1 rather than VEGFR2. Nevertheless, increased expression of VEGFR2  on tumor cells has been noted for melanoma and hematological malignancies. And, there is evidence supporting a relationship between chronic inflammation and tumor development.

 

VEGFR3 (also known as Fms-like tyrosine kinase 4, Flt4 in the mouse) is activated by the binding of VEGFC or VEGFD, once these two ligands undergo proteolytic processing (this increases their affinity to VEGFR2 and VEGFR3). In addition, hVEGFD shows similar affinity to both VEGFR2 and VEGFR3, while mVEGFD binds only to VEGFR3. During embryogenesis, VEGFR3 expression occurs in the primary vascular plexus at day E8.5. In late stages of embryogenesis, VEGFR3 is expressed in venous endothelial cells of the cardinal vein, that results in VEGFR3-expressing lymphatics. Postnatally, VEGFR3 plays an important role in lymphatic endothelial cells, but its expression is also observed in endothelial cells engaged in active angiogenesis, such as tumor vessels, in endothelial tip cells of angiogenic sprouts in the developing retina or in chronic inflammatory wounds. The receptor is also found in non-endothelial cells such as osteoblasts, neuronal progenitors and macrophages – all of which may indirectly support angiogenesis. It remains unclear if tumor cells express VEGFR3. Despite this lack of clarity, inhibiting VEGFR3 activity is associated with the arrest of tumor vascularization, resulting in decreased vascular density in several tumor models.^[1]

Since the VEGF-VEGFR pathway plays a significant role in angiogenesis, and it is widely known that VEGF is highly expressed in tumor and stromal cells, especially in the inflammatory cells of human tumors, 37 drugs that are VEGFR inhibitors are currently undergoing clinical trials for cancer therapy.

^{Drugs that inhibit VEGFR are listed below}

Cediranib (AZD2171)

Sunitinib Malate (Sutent)

Cabozantinib (XL-184)

ENMD-2076

Amuvatinib (MP-470)

AEE788 (NVP-AEE788)

Ponatinib (AP24534)

TG101209

Prednisone - The Unsung Hero of Cancer Therapy

Prednisone is used in combination with many drugs in prostate cancer therapy such as Docetaxel, Mitoxantrone, and Zytiga. It is used to potentiate the anticancer effects of these agents and to decrease side effects. However Prednisone comes with side effects of its own but these are manageable. But it also has anticancer effects of its own and explains the high overall survival of the Zytiga trial placebo arm who were all taking Prednisone plus placebo.

Docetaxel plus Prednisone has become the mainstay of prostate cancer chemotherapy and is now the standard to which other therapies are compared.

Mitoxantrone plus Prednisone is also another chemotherapy treatment option if Docetaxel treatment fails.

Zytiga plus Prednisone is now the Gold Standard of care and has set new standards in the treatment of prostate cancer.

Altogether Prednisone is the unsung hero of all these treatments and is also used for Leukaemia and Lymphoma therapy.

The molecular structure of Prednisone shows that it is a corticosteroid.

Akt Inhibitor Zytiga Combo

Clinical trials are underway on the combination of the Akt inhibitor GDC-0068 in combination with Zytiga and Prednisone. Akt is a downstream target of PI3 Kinase signalling involved in drug resistance and tumour progression. GDC-0068 is an inhibitor of Akt with an IC50 of 5 nM and a 400 mg dose has a half life of 24 hours. Chemotherapy can induce Akt activation in resistant cells and so an Akt inhibitor can overcome resistance to chemotherapy. PI3 Kinase / Akt signalling is involved in Zytiga resistance and so an inhibitor such as GDC-0068 may overcome Zytiga resistance. GDC-0068 is being developed by Genentech Roche and is a selective and potent inhibitor of Akt also known as PKB. Zytiga in combination with Prednisone is now the Standard Of Care in prostate cancer therapy and so many agents are being trialled in combination with these agents. The Zytiga GDC-0068 combo offers a rational approach to overcoming Zytiga resistance and prolonging its duration of action.

Key binding of Abiraterone (Zytiga) with the enzyme CYP17

 Structure of Prednisone (needed to overcome potassium deficiency Hypokalemia side effect of Zytiga)

Structure of Abiraterone (Zytiga)

Reduction of PSA and Tumour Volume by Zytiga

Should Xtandi Be Taken at a Lower Dose ?

The incidence of seizure from Xtandi has increased to 7 patients in the ongoing clinical trials in 800 men at a dose of 160 mg daily. Xtandi has a half life in the human body of 7 days and so its concentration will increase in the body upon daily dosing reaching potentially fatal levels in the body. The level of the drug in the human body increases daily and after 30 days the onset of seizures is observed.

Surely with a half life of 7 days this drug should be taken on a once weekly basis instead of once daily to allow the body time to metabolise it. This would reduce the cost of Xtandi dramaticaly from $ 7,450 per month to around $ 1,000 per month which makes economic sense. Weekly Xtandi dosing takes into account the long half-life of this drug and would prevent drug accumulation and so may circumvent the risk of heart seizure.

Zytiga Inhibits CYP2D6 Metabolism of Dextromethorphan

Zyiga is the trade name for Abiraterone Acetate. Once absorbed by the body this prodrug is cleaved by plasma esterases in the bloodstream to liberate free Abiraterone which inhibits the CYP17 enzyme in tumours to stop all androgen production and switches off testosterone biosynthesis. Abiraterone has been designed as a very potent and selective inhibitor of CYP17 and does not inhibit closely related CYP enzymes such as CYP19 which is used for comparison.

However during clinical development it was found that Abiraterone also inhibited the liver enzyme CYP2D6. This has important implications for drugs that are activated or deactivated by CYP2D6. Dextromethorphan is a substrate for CYP2D6 and its metabolism has been studied clinicaly in the presence of Zytiga which showed an increase in the concentration curve of Dextromethorphan showing that Zytiga decreases the metabolism of this drug and leads to prolonged Dextromethorphan exposure.

Tamoxifen is activated to its 4-hydroxy metabolite 4-hydroxy tamoxifen by CYP2D6 which is thought to be the active antiestrogenic metabolite. Tamoxifen is also N-demthylated by CYP2D6 so this enzyme plays an important role in Tamoxifen metabolism. Zytiga is undergoing clinical trials against advanced breast cancer and so its effects on decreasing CYP2D6 mediated metabolism should be taken into account. The predictions are that Zytiga will inhibit CYP2D6 and decrease the amount of 4-hydroxy tamoxifen formed and therefore Zytiga will decrease the activity of Tamoxifen.

Consequences of CYP3A4 Metabolism of Zytiga

Zytiga is metabolised by the liver enzyme CYP3A4 to an inactive metabolite that is excreted from the body. The usual half life of Zytiga is 12 hours and this rate of drug clearance is goverened by the activity of the CYP3A4 enzyme. Modulators of this enzyme will therefore affect the rate of clearance and hence activity of Zytiga.

There are 2 types of modulators of CYP3A4, inducers and inhibitors.

1. CYP3A4 inducers e.g. Rifampicin, Xtandi (Enzalutamide)

A CYP3A4 inducer will elevate the levels of this enzyme in the liver and increase the metabolism of Zytiga resulting in a shorter half life and reduced drug activity.

A clinical trial is underway to investigate the effect of Rifampicin which is a potent inducer of CYP3A4 on the metabolism of Zytiga (Abiraterone). This will look at the effect of Rifampicin exposure on the effects of a single 1000 mg dose of Zytiga and measure the levels in the plasma and rate of clearance to calculate the half life. The results of this study will have implications on the combined use of Xtandi with Zytiga since Xtandi (Enzalutamide) is also a potent inducer of CYP3A4 and may result in lower activity of Zytiga.

2. CYP3A4 inhibitors e.g. Ketoconazole, Itraconazole.
On the other hand a CYP3A4 inhibitor will block the activity of this enzyme and reduce the metabolism of Zytiga resulting in a longer half life and increased activity due to prolonged drug exposure.

Another clinical trial is investigating the effect of Ketoconazole on the metabolism of Zytiga. In theory since Ketoconazole is an inhibitor of CYP3A4 it should decrease the rate of Zytiga metabolism and prolong its duration of action. This study is investigating the effect of Ketoconazole exposure to a single 1000 mg dose of Zytiga to measure its half life. This could mean that Ketoconazole may be succesfully combined with Zytiga to increase its activity and would mean that lower doses of both drugs could be used together.

Yervoy Zytiga Combo

It had to happen - combine monoclonal antibody therapy with Zytiga antihormonal therapy. Yervoy (Ipilimumab) is a monoclonal antibody aimed at the CTLA-4 antigen which reactivates the Cytotoxic T-Lymphocytes to recognise and destroy tumour cells. Zytiga is now established as a standard of care in prostate cancer therapy and so many new drugs with potential against prostate cancer are now compared with Zytiga therapy.

Yervoy in combination with Zytiga respresents an interesting combination since the Zytiga will starve the tumour of androgenic hormones whilst the Yervoy will stimulate the killer T-Cells to attack the tumour and should work against tumours that have metastasized. A clinical trial is currently underway to test the safety and efficacy of this combination with oral Zytiga being taken daily along with Prednisone, and Yervoy administered IV every 3 weeks for 4 cycles. The effect on PSA response will be examined along with progression free survival time.