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Chemotherapy drugs fall into categories according to to the way they wage war against cancer.  Cytotoxic drugs are the killers, and cytostatic drugs are the agents of cell infertility.   

CYTOSTATIC DRUGS

When tumors take residence inside the body, they feed themselves by absorbing nutrients through an ever-growing network of blood vessels.  The growth of these rogue blood vessels is called “angiogenesis;” interference with their growth is “angiogenesis inhibition.” Tests are now being conducted to determine whether drugs like halidomide, interferon, bevacizumab (Avastin), cilengitide (EMD 121974), lenalinomide (Revlimid), AZD 2171 (cedirinib) and can stop the growth of a tumor’s blood supply.

CYTOTOXIC DRUGS

How do I kill thee?  Let me name the ways.  Alkylating agents seduce and trap tumor cells by bonding with their DNA strands.  This prevents the tumors from reproducing. Carboplatin, cisplatin, cyclophosphamide, and temozolomide (Temodar) are examples of alkylating agents. The alkylating agents known as nitrosoureas, specifically Carmustine (BCNU) and lomustine (CCNU), are even more insidious; they prevent tumor cell reproduction by preventing the tumor cells from repairing themselves.  

Antimetabolites such as Methotrexate(MTX) block tumor cells from manufacturing the enzymes they need to grow.  The drug Rapamycin is part of a subclass of antimetabolites called anti-Tumor Antibiotics, which in addition to acting against enzymes can also interfere with the environment around the tumors.   

Hormones (derived from animals) and mitotic inhibitors (derived from plants) wage war against cancer by interfering with protein production inside tumor cells.  Tamoxifen, for example, is a hormone used to treat breast cancer.  Etoposide (VP-16), paclitaxel (Taxol) and vincristine are examples of mitotic inhibitors.

When human cells divide, they go through a series of steps called the cell cycle.  First they manufacture the proteins and enzymes they will need during reproduction.  Second, they duplicate their own DNA.  Third, they separate the DNA into sets – one for each new cell.    Cancer cells, like healthy cells, reproduce with the cell cycle.  They are different only because of the high speed at which they divide, and the mutations inside their DNA that make them harmful to the body.  

Cytostatic chemo drugs stop the growth of cancer cells by throwing the cell cycle off course.    They can do this in two ways.   Traditional chemo interferes with cells as they divide.   Biologic or targeted agents, a newer form of chemo, stop cells from beginning to divide.       Drugs that use only one of these methods -- traditional or biologic -- are called "cell-cycle specific drugs" because they act only during one phase of the cycle.   "Non cell-cycle specific" drugs use both methods and can stop the cycle at any point.    When used together, cell-specific and non-cell-specific drugs can stop a larger number of tumor cells from dividing.   

The inherent problem with most chemotherapy is that it cannot distinguish between the cell cycles of fast-dividing tumors and those of healthy fast-dividing cells.  Chemo drugs interfere with both.  Hence, the side effects of chemotherapy are virtually inevitable.  A successful chemo treatment must be designed with the utmost care to diminish these side effects.   It must also deliver the right drugs to exactly the right areas of the body, where they will hit tumors as hard as possible while minimizing damage to healthy cells.   

In order to control where the drug goes and keep side effects to a minimum, most chemotherapy is delivered locally -- to specific parts of the body, through injection rather than pills.   There are several ways to do this.  
Intracavitary delivery injects the treatment into the cavity left after a tumor has been surgically removed.  Interstitial delivery sends a brain tumor drug directly into brain tissue.    Intrathecal delivery injects drugs into the space between the meninges -- the membranes surrounding the central nervous system.  

Intratumoral delivery injects drugs directly into the tumor.   One subtype of this direct-to-tumor approach is "Convection enhanced delivery, " perhaps the most elegant chemotherapy technique of all, which injects drugs directly into the tumor using controlled flow or the force of gravity“• into a ventricle, called “intraventricular” delivery

Isolated infusion uses tubes to deliver chemotherapy directly into the liver or lung. Isolated limb perfusion is used often with melanoma, a form of skin cancer.  It puts anticancer drugs directly into the blood of an arm or leg by temporarily blocking the flow of blood to and from the limb with a tourniquet. This allows the person to receive a high dose of drugs in the area where the cancer occurred.  

While all of the above are examples of local delivery of tumors, many forms of chemotherapy use systemic delivery.  Patients swallow them as pills, or get them as injections into the artery (intra-arterial delivery), muscle (intramuscular delivery), vein (intravenous delivery) or skin (Subcutanous or SubQ delivery).  The drugs then travel through the bloodstream to all parts of the body and the brain. Two medicines for brain tumors, Lomustine (CCNU) and temozolomide (Temodar), are examples of systemic drugs.  They enter the body as pills, and travel via the blood until they cross the blood brain barrier and invade brain tumor cells. Medicines such as melphalan, busulfan or capecitabine (which treat, among other things,  ovarian cancer, colorectal cancer, and leukemia)    also arrive in the body as simple pills and are delivered throughout the bloodstream.  

* This article is based on the information at  http://www.chemotherapy.com, http://en.wikipedia.org/wiki/Chemotherapy, http://www.nih.gov, http://www.webmd.com and http://www.cancer.org