Medical Pharmacology Topics
Cancer
ocurs when normal cells are "transformed" into tumorigenic cells.
Normal cells hve a limited life span, i.e. there is a finite number of population
doublings when they are cultured in vitro. They also exibit contact inhibition,
so will stop growing when a monolayer is achieved and all cells come in contact
withtheir neighbors. Normal cells also depend on growth factors to promote growth
and cell division, otherwise they will remain in G0. Most normal cells are difrficult
to culture in vitro and they require a hard substrate to grow on, like plastic
or glass. Normal cells will not grow in soft agar.
Transformed Cells
Cells may be transformed by DNA viruses, chemicals or by DNA isolated from tumor cells and transfected into normal cells. Transformed cells do not exibit contact inhibition and do not stop growing when in contact with their neighbors. A complete depletion of nutrients is necessary to slow or stop cell division. Transformed cells are said to be "inmortalized", because the cels and their progeny will grow indefinitely after a culture line is extablished in vitro, without exibiting aging or senescence. They are also less dependent opon exogenous growth factors and may produce their own tumor growth factors (TGF). They may stimulate their own growth through autocrine stimulation.
Transformed cells may be nchorage-ndependent and capable of growing in soft agar or formin tumor when injected into experimental (immunosupressed) animals. Not all transformed cells are ncesarily tumorigenic. Tumorigenic cells have all the characteistics of transformed cells and also have two or more oncogenes activated or overexpressed. The most common featue of tumor cells is uncontroled growth.
The transfection of foreing DNA that transforms cells usually causes the activation of one oncogene. The current concept of oncogenes suggestas that there are inmortalizing oncogenes that ca transform a normal cell into an "inmortalized" cell, and there are tumorigenic oncogenes whose expression allows the cell to be invasive and cause tumors. Thus, for a cell to be tumorigenic requires activation of one inmortalizing oncogene and one tumorigenic oncogene.
Antitumor
Agents
There are 5 major classes of antitumor agents according to their mechanism of action:
The adduct-forming agents can be subdivided into two major groups: alkylating agents and platinum compounds. The alkylating agents can be further subdivided into four groups: nitrogen mustards, chloroethylnitrosoureas, monofunctional alkylating agents and AZQ.
There are at least 6 targets for antitumor agents:
In general, most antineoplastic drugs affect cells during S phase. Some agents are cell-cycle specific, killing cells only if they are not in G0, these include the antimetabolites, some intercallating agents (ex. bleomycin), and antimitotics. Other drugs are not cell-cycle specific and may kill cells in G0, although cycling cells are more succeptible: adduct-forming agents and intecallating agents.
Since cycling cells are more succeptible to antitumor agents, most toxic side effects are the result of the agent's effect on normally dividing cells. Toxicities common to all cemotherapeutic agents include:
Second neoplasms may also occur due to the drug's mutagenic effects on normal cells. Other side effecys due to toxicity to non-dividing cells, such as the emetic center of the brain, include nausea/vomiting, anorexia and fatigue.
There is a different type of antineoplastic agent used to treat chronic myelogenous leukemia (CML), ST1571. These agent does not attack a tumor but a deffective kinase produced by the malignancy. It has been established that the Bcr-Abl protein, created as a result of a chromosome translocation, is the cause of CML. Bcr-Abl functions as a constitutively active tyrosine kinase. ST1571 is a Bcr-Abl kinase inhibitor nd has shown remarkable activity in all phases of CML. it functions by blocking the binding of ATP to Bcr-Abl.
In addition to inhibiting Bcr-Abl, ST1571 inhibits the PDGF-R and c-kit tyrosine kinases, thus ST1571 may be useful in other diseases were inases are activated. In recent studies, ST1571 produced dramatic clinical responses in patients with Bcr-Abl positive CML and c-kit positive gastrointestinal stromal tumors. Still, mechanism of resistance to ST1571 may develop: overexpression of Bcr-Abl, mutations in the STI571 binding site of Bcr-Abl, and overexpression of P-glycoprotein.
Apoptosis
Cell renewal is an essential component of homeostasis in all tissues, a delicate balance between cell proliferation and cell death that maintains aDequate tissue volume. But tumors can increase cell number by increasing proliferation and inhibiting cell death.
Programmed cell death, or apoptosis, is a pathway of biochemical and morphological steps for the deletion of cells from normal tissue. Apoptosis is a genetically regulated set of events in which nuclear DNA is fragmented between nucleosomes and the rest of the cell is dissasembled into apoptotic bodies that are later phagocytized. It is a cytoplasmic event mediated by proteases, different from necrosis in which the cell membrane is destroyed first, followed by destruction of internal structures.
Cancer therapy causes an induction of the program of apoptotic cell death. Antitumor agents act on many different initial biochemical events, but the end result appears to always be apoptosis. Blocking the apoptosis program would provide resistance to the therapy. Tumur cells from a wide variety of human malignancies have deceased ability to undergo apoptosis. This is most apparent in metastatic tumors.
Molecules involved in apoptotic pathways include the Bcl-2 family of proteins, the cysteine proteases or caspases, cell surface death receptors and their ligands, and the p53 tumor suppressor protein.
The Bcl-2 family of proteins include Bcl-2, Bax, and Bcl-x. Overexpression of Bcl-2 prevents cells from initiating apoptosis in response to a number of stimuli. Bcl-2 expression has been associated with a poor prognosis in prostate and colon cancers and neuroblastoma.
Overexpression of Bax accelerates apoptotic death in response to many death-promoting stimuli. Bax also counters the death repressive activity of Bcl-2.
The long fom of Bcl-x (Bc-xL) represses cell death, while the short form (Bcl-xS) favors cell death. In tuor cells, Bcl-2 and Bcl-x have been found to confer resistance to cell death in response to many chemotherapeutic agents.
The interaction between Bcl-2 family proteins either prevents or promotes caspase activation. Caspases proteoliticlly digest some of the key cellular proteins and promote cell death.
Cell surface receptors and teir ligands include Fas and its receptor, tumor necrosis factor alpha (TNFa) and its receptors), and TNFa receptor-related apoptosis-inducing ligand (TRAIL) and its receptors. Binding of these ligands to their receptors will also activate the caspases.
The p53 tumor suppresor protein is required for cells to initiate the apoptotic response to genotoxic damage. p53 ma directly activate death-promoting agents such as Bax, or downregulate survival genes such as Bcl-2. Therefore G1 arrest and apoptosis seem to be alternative p53 outcomes.
Two pathways involved in apoptosis are the mitochondrial pathway and the cell surface receptor pathway. In the mitochondrial pathway, apoptotic stimuli triggers the relese of cytochromo c, certain caspases, apoptosis-inducing factor (AIF), Smac/DIABLO, and other proapoptotic factors from the intermenbrane space to the cytosol. Once released, cytochrome c, together with ATP, binds to apoptotic proteinase ctivating factor (Apaf-1), and this complex promotes activation of procaspase-9, which in turn activates procaspases -2, -3, -6, -8, and -10.
In the cell surface receptor pathway, Fas, TNFa or TRAIL interaction with their respective receptors transduce death signals to activate procaspase-8, an initiator caspase which can activate the downstream caspases, including caspase-3.Caspase-8 also cleaves proapototic Bcl-2 family member Bid, which induces mitochondrial cytochrome c release, thus linking the two pathways.
Drug Resistance
40% to 45% of cancers are either intrinsically resistant or aquire resistance to chemotherapy during the course of treatment. This is the major factor in our relative lack of success in treating most solid tumors. Tumors may become resisant to chemotherapy by many different mechanism, but the clinical relevance of the known mechanisms remains unclear.
Pharmacological factors involved in relative drug resistance include: variations in drug bioavailability, metabolism or elimination; prescence of tumor in inaccesible sites; excess host toxicity; limited drug diffusion; differences in cell kinetics (some drugs only act on proliferating cells); and variations in salvage factors (?).
Drug resistance mechanisms include: reduced intracellular concentrations (increased sequestration or efflux), increased detoxification (decreased activation or increased deactivation), increased DNA repair, alteration in drug targets o inactivation of apoptotic pathways.
Increased DNA repair activity helps cancer cells survive chemtherapeutic attack by aduct-forming agents like the chloroethylnitrosoureas or platinum compounds. For example, chloroethylnitrosoureas induce alkylation at guanine O6. The enzyme O6 methylguanine methyltransferase (MGMT) removes alkylating moieties from the O6 position of guanine. Therefore increased expresion of MGMT causes resistance to chloroethylnitrosoureas.
Alteration or mutation of the enzyme targets of antimetabolite drugs decrease the affinity of the drug for the enzyme and thus provides resistance to therapy. For example, methotrexate inhibits dihydrofolate reductase (DHFR) activity. Mutations in DHFR that reduce its affinity ofr methotrexate while retaining catalytic activity will confer resistance against methotrexate therapy.
Alterations or decreased expression of the enzyme required to activate a drug wll cause resistance. For example, 6-mercaptopurine needs to be converted to 6-thioinosine-5-phosphate by hypoxanthine guanine phosphoribosyltransferase (HGPRT). Decreased expression of HGPRT will confere resistance aginst 6-mercaptopurine therapy.
An example of increased drug inactivation is the conjugation of alkylating agents by gluthatione, spontaneously or mediated by glutathione S-transferase (GST). Glutathione-conjugated drugs can be eliminated by the MRP transporter.
A reduction of active intracellular drug concentation can also be achieved by increased drug sequestration by methallothionen. Humans have 10 metallothionein genes, and elevated expression of these genes causes resistance to the platinum dduct forming agents.
Some drug-resistant cancer cells express a high level of one or more drug efflux pumps on their plasma membran, keeping low intracellular concentrations of several drugs. Three such efflux pumps have been indentified: multidrug rasistance or P-glycoprotein (MDR/Pgp), the multidrug resistance protein (MRP), and the breast cancer resistance protein (BCRP/MXR). All of these belong to the ABS superfamily of ATPase transporters.
The transmembrane domains of Pgp may form a channel. Drugs ae transpoted thru this channel either as-i or bound to a protein. ATP is hydrolized at the ATP binding domains (NBD1 and NBD2), which function as an ATPase to provide energy. There are two types of Pgp: MDR1 and MDR2. While MDR1 is known to provide multidrug resistance, the function of MDR2 is not known.
There
9 types of the Multidrug Resistance Protein (MRP). Most of them provide multi-
or limited drug resistance, one is an organic anion transporter, and for at
least 3 variants the function is unknown. The Breast Cancer Resistance Protein
(BCRP/MXR) may function similarly to Pgp or MRP but as a dimer.
There is an overlaping substrate specificity for the three known drug resistance proteins. While while some drugs are transported by only one o two, others can be transported by all three resistance proteins. or example, methotrexate is only transported by MRP and paclitaxel by Pgp, while vinblastine is transported by both Pgp and MRP. Mitoxantrone is transported by Pgp and BCRP. Doxorubicin is transported by all three proteins.
Measurement of resistance protein expression in tumors is useful in cancer treatment. Both disease erradication and overall survival is significantly reduced in patients with Pgp-positive tumors. Pgp enhances the invasiveness of the tumor, possibly by influencing cell locomotion and cellular adhesion.
Drug-resistant tumors may be chemosensitized using drugs that compete with the chemotherapy agent for Pgp transport. Some of these agents are verapamil, quinidine, tamoxifen, cyclosporin A and VX-710. Ideal criteria for a chemosensitization agent include:
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