O6-alkylating agent: chemical agents that alkylate (eg, methylate) the O6 of guanine in DNA and are used in chemotherapy. Examples are 1,3-bis(2-chloroethyl)-1-nitrosourea and temozolomide.
oblimersen (Genasense): antisense oligonucleotide inhibiting Bcl-2
mRNA translation and therefore protein production. This is an example of a genetic inhibitor of Bcl-2 currently in clinical trial.
OctreoScan Scintigraphy: an in vivo scintigraphy procedure developed as a result of the high and frequent expression of somatostatin receptors in endocrine tumors that is currently applied in clinical practice. 111In-DTPA-D-Phe1-octreotide (OctreoScan) is injected and then anterior and posterior planar localized images of the head, chest, abdomen, and pelvis are acquired (together with single-photon emission computed tomography [SPECT] imaging on targeted
anatomic regions). Positive signal is defined as when higher than or equivalent to the normal liver tracer uptake occurs. Negative signal is defined as no tracer fixation or lower than normal liver activity of the corresponding radiologic abnormalities considered a metastasis or the primary tumor. Indications for OctreoScan scintigraphy are staging of known primary endocrine tumors before treatment (is there a metastasis or not?), staging of endocrine tumor after
resection (is there a recurrence/metastasis or not?), pretherapeutic investigation before internal radiotherapy using high-energy radiolabeled somatostatin analogs (positive fixation of targeted tumors is required).
off-label: the use of drugs outside of their US Food and Drug Administration–approved indications.
olaparib: a highly potent, orally active poly (ADP-ribose) polymerase inhibitor in clinical development for the treatment of cancer. Formerly known as AZD2281 and KU-0059436.
oligoastrocytoma: a peculiar type of oligodendroglial tumor,
with mixed features of astrocytoma, but at least 25% of oligodendroglial elements. According to the WHO classification, oligoastrocytomas correspond to grade II tumors.
oligodendroglioma: approximately 5% of all intracranial gliomas. According to the WHO classification, oligodendrogliomas correspond to grade II tumors. These gliomas are infiltrating and low in mitotic activity but recur and sometimes tend to progress to anaplastic tumors.
Usually characterized by chemo- and radiosensitivity and by the typical chromosomal loss of heterozygosity of 1p and 19q.
oligomerization: the forming of a compound (polymer) by
the combination of relatively few smaller molecules (monomers).
oligometastatic/limited metastatic disease: an intermediate disease state that exists between only localized disease and widespread metastases/diffuse disease dissemination. The term often applies to single or fewer than three to five sites of metastatic deposits in single or limited multiple organs (ie, brain, liver, bone, lung).
oligonucleotide arrays: high-density arrays containing in situ synthesized antisense oligonucleotides (an average of 25 bases long) matching thousands of mRNA transcripts sequences.
oligonucleotide microarray chips: see cDNA microarray.
oligonucleotide microarray gene expression assays: technology that allows for identification of gene expression levels in a biologic sample. cDNAs or oligonucleotides, each representing
a given gene, are immobilized on a small chip or nylon membrane and tagged; they then serve as probes that will indicate whether they are expressed in biologic samples of interest. Thus, the simultaneous expression of thousands of genes can be monitored at the same time.
omalizumab: a recombinant anti-immunoglobulin E antibody. Omalizumab was developed to bind immunoglobulin E, which is generated during allergic reactions, and block the underlying symptoms of allergic asthma.
Omi/HtrA2: released, along with Smac and cytochrome c, from the mitochondria into the cytoplasm upon disruption of the outer mitochondrial membrane during apoptosis. Omi/HtrA2 also induces apoptosis in a caspase-dependent manner.
oncogene addiction: the phenomenon in which some cancers that contain genetic, epigenetic, and/or chromosomal abnormalities remain dependent on one or more genes for both maintenance of the malignant phenotype and cell survival.
oncogene dependence: the phenomenon in which some tumor cells require expression of a specific oncogene for their survival despite the fact that normal cells from which the tumor originated were viable in the absence of the oncogene. See oncogene addiction.
oncogenes: normal cellular genes whose protein products may function in any number of critical molecular pathways (and are referred to as proto-oncogenes when functioning in a nonmutated state) that are subject to activating, or gain-of-function mutations, which stimulate tumorigenesis.
oncogenomics: study of the cancer genome using high-throughput technologies.
Oncotype DX classifier: a classifier used to estimate risk of recurrence for patients with node-negative estrogen receptor–positive primary breast cancer receiving tamoxifen. Components of this classifier are expression levels (based on reverse transcriptase polymerase chain reaction optimized for use with paraffin embedded formalin fixed tissue) of selected genes that were initially identified as prognostic based on published microarray studies. The completely specified classifier was externally validated based on archived specimens from independent studies.
optimism: the decrease in model performance in new patients compared with performance in the sample under study.
organic anion-transporting polypeptide family (OATP): represents a family of the organic anion transporter, which facilitates the Na ion independent transport of a variety of
compounds including bile acids, bilirubin, steroid hormone conjugates, thyroid hormones, prostaglandins, clinically used drugs, and toxins. OATPs are transmembrane glycoproteins, expressed in various cells and tissues.
ornithine decarboxylase: the first enzyme in polyamine biosynthesis. Ornithin decarboxylase catalyzes the rate-limiting decarboxylation of L-ornithine to diamine purescine. Its activity is under the regulation of an inhibitory protein called antizyme, which binds to the enzyme, inhibits its activity, and targets it for proteasomal degradation. Polyamines are essential for normal cell growth and proliferation.
oropharyngeal carcinoma: a carcinoma arising from the middle part of the pharynx behind the mouth and includes the back one third of the tongue, the soft palate, the side and back walls of the throat, and the tonsils. This typically includes the base of tongue, soft palate, uvula, tonsils, and pharyngeal walls.
osteoblast: cells responsible for synthesis and mineralization of bone during bone formation and remodeling.
osteoclast: a cell that breaks down bone and is responsible for bone resorption. Osteoclasts are large multinucleate cells that differentiate from macrophages.
ovarian failure: women who either never experienced spontaneous menses or experienced spontaneous menses but had cessation of menses prior to age 40 years.
overall survival: the duration between random assignment and death.
oxidative stress: caused by an imbalance between reactive oxygen species and the bodys ability to detoxify reactive intermediates. Oxidative stress can result in cellular damage as well as programmed cell death.