Cancer Radio Therapy

The demise of cells by programmed cell death referred to as apoptosis, a Greek

word that means “dropping off“ or “falling off“ as in leaves from a tree, has been

recently a topic of intense interest in biomedical sciences. Apoptosis is a well-

defined sequence of morphological changes of cells that shrink and condense and

then fragment, releasing small membrane-bound apoptotic bodies, which are

phagocytosed by other cells. Importantly, the intracellular constituents are not

released into the extracellular milieu where they might have deleterious effects on

neighboring cells. On the contrary, cells that die in response to tissue damage or

other reasons exhibit very different morphological changes generally called

necrosis. The cells that undergo this process swell and burst, releasing their

intracellular contents, which can damage surrounding cells and often cause

inflammation. Apoptosis refers to a particular morphology in which a chromatin

condenses or coalesces to a heterochromatin in one or more masses in the nucleus.

It usually settles along still-intact nuclear membrane referred to as margination of

the chromatin. One of the essential functions of apoptosis is the elimination of cells

in which DNA damages, faulty proliferation or improper adhesion to extracellular

matrix that cannot be repaired. In cancer cells, the mechanism of apoptosis

induction is broken. Therefore, more and more ideas and hypotheses for selective

inducing apoptosis in cancer cells are tested in a growing number of laboratories all

over the world. The subject of programmed cell death has been recently discussed

in almost 80 000 publications. As it is known, cell apoptosis may be induced by

various stress factors (e.g. hypoxia, expression of oncogenes, mutations, DNA

damages). On the other hand, apoptosis may be induced via internal or external

signals, for instance proteins. Some of such endogenous and exogenous

proapoptotic proteins have been found and described. Their genes may be used in

modern anticancer therapies.

For example, introducing into cancer cells proapoptotic genes as Bax, Bcl-X5 or

E2F-1 significantly increases induction of apoptosis. Some clinical trials concern

therapeutic application of a 121-amino acids apoptin originated from chicken

anemia virus (CAV). Recent data suggest that apoptosis induced by this protein

involves caspases, a family of cysteinyl aspartate-specific proteinases. In vitro

results show that apoptin is very active against cancer cells without inducing toxicity

to normal cells. This tumor-specific effect may be explained by the nuclear

localization of the protein in tumor cells required for its action. Moreover, apoptin is

equally active, such as p53-mutant, Bcl-2-overexpressing or BCR-ABL-expressing

tumor cells. Other investigations showed that E4orf4 induces apoptosis in cancer

cells by linking with 2A (PP2A) phosphatase. Unfortunately, induction of apoptosis

by introducing genes encoding proapoptotic proteins has been little known. One

possible mechanism is associated with destruction of mitochondrial membranes

and, in consequence, disturbing electrons transport, oxidative phosphorylation and

ATP synthesis. Finally, the cell dies but the death is slightly different than that

during typical apoptosis induced by caspases due to prolonged time of this process.

Proapoptotic proteins cannot be directly introduced to cancer cells because there

are no specific receptors. They are transported through membranes in complexes by

special fusion proteins called ligands.

Other method is introducing them as genes by vectors and this approach has been

already successfully applied. Clinical trials are presently underway to test efficiency

of new apoptosis-triggering drugs. A large number of adenoviral agents are being

constructed, including replication-incompetent and replication-selective oncolytic

adenoviruses. One of them is ONYX-015, a replication-competent virus genetically

engineered to selectively replicate in and lyse p53-deficient cancer cells. Other

agent, INGN 201, was shown to deliver a p53 expression. Preclinical studies in

human cell lines and animals with head and neck cancers have shown that the p53

gene is transcribed and translated into p53 protein. Respectively, 5% and 58% of

patients receiving three intratumoral injections of INGN 201 in conjunction with

radiation therapy for over 6 weeks were shown to have achieved complete and

partial responses. Other example may be a gene encoding the proapoptotic Vpr

protein that was successfully transferred into cancer cells by the HIV-1 virion. These

agents are introduced by intravascular infusion or intratumoral or epitumoral

injections. An example of a target therapy against cancer is an intravenous

administration of liposomal form of tretinoin (ATRA). Treatment of acute

promyelocytic leukemia (APL) with ATRA alone or in combination with chemotherapy

results in an almost complete remission rate as high as 85% to 95%.

Other proapoptotic anticancer therapeutics is Genasense developed by the Genta

Company. Genasense is a phosphothioate oligonucleotide consisting of 18 modified

DNA bases. First, the single-stranded DNA molecule must be incorporated into a

cancer cell and then target the mRNA by having a complementary sequence to it.

This drug inhibits the production of a protein known as Bcl-2 that is widely

expressed in many types of cancer. This up-regulation of Bcl-2 blocks the release

of cytochrome C from the mitochondria thereby preventing apoptosis. Furthermore,

Bcl-2 appears to be a major contributor to both inherent and acquired resistance to

current anticancer treatments. By inhibiting production of Bcl-2, Genasense enables

the cancer cells to be killed by apoptosis when treated with current state of the art

therapy. Interesting apoptosis-inducing drug is Velcade jointly developed by NCI

and Millenium Pharmaceuticals. Activity of Velcade is mainly associated with

reversible inhibition of the proteasome and building up many proteins including

BAX. In the normal cells, the BAX protein induces apoptosis by blocking the activity

of Bcl-2. When BAX level increases, BAX inhibition of Bcl-2 also increases and the

cells undergo apoptosis. Non-clinical studies have demonstrated that cancer cells

are more sensitive to the effects of the proteasome inhibition than normal cells.

Selected references

Adachi, S.L.L., Carson, D.A., Nakahata, T., 2004. Apoptosis induced by molecular

targeting therapy in hematological malignancies. Acta Haematologica 111, 107

-123.

Ferreira, C.G., Epping, M., Kruyt. F.A.E., Giaccone, G., 2002. Apoptosis: Target of

Cancer Therapy. Clinical Cancer Research 8, 2024-2034.

Ghobrial, I.M., Witzig, T.E., Adjei, A.A., 2005. Targeting Apoptosis Pathways in

Cancer Therapy. CA: A Cancer Journal for Clinicians 55, 178-194.

Hengartner, M.O., 2000. The biochemistry of apoptosis. Nature 407, 770-776.

Lowe, S.W., Lin, A.W., 2000. Apoptosis in cancer. Carcinogenesis 21, 485-495.

Tamm, I., Dorken, B., Hartmann G., 2001. Antisense therapy in oncology: new hope

for an old idea? Lancet 358, 489-197.

Tamm, I., Schriever, F., Dorken, B., 2001. Apoptosis: implications of basic research

for clinical oncology. Lancet Oncology 2, 33-42.