activity. fluoroquinolones have proven to be effective on a wide array of microbial targets, with some third and fourth generation drugs possessing both anti-gram positive and anti-anerabic capabilities. currently, the us food and drug administration (fda) has updated the public on eight new-generation fluoroquinolones: moxifloxacin, delafloxacin, ciprofloxacin, ciprofloxacin extended-release, gemifloxacin, levofloxacin, and ofloxacin. it was observed that the new fluoroquinolones can cause hypoglycemia, high blood pressure, and mental health effects such as agitation, nervousness, memory impairment and delirium. although quinolones are successful as antibiotics, their effectiveness is limited due to accumulation of small mutations and multi-drug efflux mechanisms, which pump out unwanted drugs out of the cell. in particular, smaller quinolones have shown to bind with high affinity in the multi-drug efflux pump in escherichia coli and staphylococcus aureus. despite quinolones ability to target topii, they can also inhibit topiv based on the organisms and type of quinolone. additionally, the discovery of mutations in the gyrb region is hypothesized to cause quinolone-based antibiotic resistance. specifically, the mutations from aspartate (d) to asparagine (n), and lysine (k) to glutamic acid (e) are believed to disrupt interactions, leading to some loss of tertiary structure. mechanically, the since disproven, shen et al. (1989) model of quinolone inhibitor binding proposed that, in each dnagyrase-dna complex, four quinolone molecules associate with one another via hydrophobic interactions and form hydrogen bonds with the bases of separated, single stranded segments of dna. shen et al. based their hypothesis on observations regarding the increased affinity and site specificity of quinolone binding to single stranded dna compared to relaxed double stranded dna. a modified version of the shen et al. model was still regarded as a likely mechanism in the mid to late 2000s, but x-ray crystallography-based models of inhibitor-dna-topii complex stable intermediates developed in 2009 have since contradicted this hypothesis. this newer model suggests that two quinolone molecules intercalate at the two dna nick sites created by topii, aligning with a hypothesis proposed by leo et al. (2005). intercalating poison topii inhibitors have two main identification: poisons and catalytic inhibitors. topii poisons are characterized by their ability to create irreversible covalent bonds with dna. furthermore, topii poisons are divided into two groups: intercalating or non-intercalating poisons. the anthracycline family, one of the most medically prevalent types of intercalating poisons, are able to treat a variety of cancer due to its diverse derivations and are often prescribed in combination with other chemotherapeutic medications. the first anthracycline (doxorubicin) was isolated from the bacteria streptomyces peucetius in the 1960s. anthracyclines are composed of a core of four hexane rings, the central two of which are quinone and hydroquinone rings. a ring adjacent to the hydroquinone is connected to two substituents, a daunosamine sugar and a carbonyl with a varying side chain. currently, there are four main anthracyclines in medical use: doxorubicin daunorubicin (doxorubicin precursor) epirubicin (a doxorubicin stereoisomer) idarubicin (a daunorubicin derivative) idarubicin is able to pass through cell membranes easier than daunorubicin and doxorubicin because it possesses less polar subunits, making it more lipophilic. it is hypothesized that doxorubicin, which possesses a hydroxyl group and a methoxy group not present in idarubicin, can form hydrogen bonding aggregates with itself on the surface of phospholipid membranes, further reducing its ability to enter cells. despite the success of these poisons, they have been shown that interaction poisons have a few limitations including 1) little inhibitor success of small compounds 2) anthracyclines' adverse effects such as membrane damage and secondary cancers due to oxygen-free radical generation 3) congestive heart failure. the harmful oxygen free radical generation associated with the use of doxorubicin and other anthracyclines stems, in part, from their quinone moiety undergoing redox reactions mediated by oxido-reductases, resulting in the formation of superoxide anions, hydrogen peroxide, and hydroxyl radicals. the mitochondrial electron transport chain pathway containing nadh hydrogenase is one potential instigator of these redox reactions. the reactive oxygen species produced by interactions like this can interfere with cell signaling pathways that utilize protein kinase a, protein kinase c and calcium/calmodulin-dependent protein kinase ii (camkii), a kinase integral in controlling calcium ion channels in cardiomyocites. non-intercalating poisons another category of topii poisons is known as non-intercalating poisons. the main non-intercalating topii poisons are etoposide and teniposide. these non-intercalating poisons specifically target prokaryotic topii in dna by blocking transcription and replication. studies have shown that non-intercalating poisons play an important role in confining topii-dna covalent complexes. etoposide, a semi-synthetic derivative of epipodophyllotoxin is commonly used to study this apoptotic mechanism and include: etoposide teniposide both etoposide and teniposide are naturally occurring semi-synthetic derivatives of podophyllotoxins and are important anti-cancer drugs that function to inhibit topii activity. etoposide is synthesized from podophyllum extracts found in the north american may apple plant and the north american mandrake plant. more specifically, podophyllotoxins are spindle poisons that cause inhibition of mitosis by blocking mitrotubular assembly. in relation, etoposide functions to inhibit the cell cycle progression at the pre-mitotic stage (late s and g2) by breaking strands of dna via the interaction with dna and topii or by the formation of free radicals. etoposide has shown to be one of the most active drugs for small cell lung cancer (sclc), testicular carcinoma and malignant lymphoma. studies have indicated that some major therapeutic activity for the drug has been found in small cell bronchogenic carcinoma, germ cell malignancies, acute non-lymphocytic leukemia, hodgkin's disease and non-hodgkin's lymphoma. additionally, studies have shown when treated with etoposide derivatives there is an anti-leukemic dose response that differ compared to the normal hematopoietic elements. etoposide is a highly schedule-dependent drug and is typically administered orally and recommended to take twice the dosage for effective treatment. however, with the selective dosage, etoposide treatment is dose limiting proposing toxic effects like myelosupression (leukopenia) and primarily hematologic. furthermore, around 20-30% of patients who take the recommended dosage can have hematologic symptoms such as alopecia, nausea, vommitting and stomatitis. despite the side effects, etoposide has demonstrated activity in many diseases and could contribute in combination chemotherapeutic regimens for these cancer related diseases. similarly, teniposide is another drug that helps treat leukemia. teniposide functions very similarly to etoposide in that they are both phase specific and act during the late s and early g2 phases of the cell cycle. however, teniposide is more protein-bound than etoposide. additionally, teniposide has a greater uptake, higher potency and greater binding affinity to cells compared to etoposide. studies have shown that teniposide is an active anti-tumor agent and have been used in clinical settings to evaluate the efficacy of teniposide. in a study performed by the european organization for the research and treatment of cancer (eortc) and lung cancer cooperative group (lccg), the results of toxicity of teniposide indicated hematologic and mild symptoms similar to etoposide. however, the study found that the treatment outcome for patients with brain metastasis of sclc had low survival and improvement rates. mutations although the function of topii poisons are not completely understood there is evidence that there is differences in structural specificity between intercalating and non-intercalating poisons. it is known that the difference between the two classifications of poisons rely on their biological activity and its role in the formation of the topii-dna covalent complexes. more specifically, this difference occurs between the chromophore framework and the base pairs of dna. as a result of their structural specificity, slight differences in chemical amplification between antibiotics are seen. thus, this provides explanation on why theses drugs show differences in clinical activity in patients. despite the difference in structural specificity, they both present mutations that result in anticancer drug resistance in relation to intercalating poisons, it has been found that there are recurrent somatic mutations in the anthracyclines family. studies have shown that in dna methyltransferase 3a (dnmt3a) the most frequent mutation is seen at arginine 882 (dnmt3ar882). this mutation impacts patients with acute myeloid leukemia (aml) by initially responding to chemotherapy but relapsing afterwards. the persistence of dnmt3ar882 cells induce hematopoietic stem cell expansion and promotes resistance to anthracycline chemotherapy. while there has not been enough research on specific mutations occurring among non-intercalating poisons, some studies have presented data regarding resistance to etoposide specifically in human leukemia cells (hl-60). r. ganapathi et al. reported that the alteration in activity of topii as well as a reduced drug accumulation effect tumor cell resistance to epipodophyllotoxins and anthracyclines. it has been proposed that the level of topii activity is an important determination factor in drug sensitivity. this study also indicated that hypophosphorylation of topii in hl-60 cells when treated with calcium chelator (1,2-bis-(2-aminophenoxy)ethane-n,n,n',n'-tetraacetic acid acetoxymethyl ester) resulted in a > 2-fold reduction in etoposide-induced topii-mediated dna cleavable complex formation. scientists have indicated that this could be a plausible relationship between etoposide drug resistance and hypophosphorylation of hl-60 cells. additionally, a study reported by yoshihito matsumoto et al. showed an incidence of mutation and deletion in topiiα mrna of etoposide and m-amsacrine (mamsa)-resistant cell lines. topiiα showed a decrease in activity and expression and an increase of multidrug resistance protein (mrp) levels. as a result, this diminished the intracellular target to etoposide and other topii poisons. furthermore, it was found that phosphorylation of topiiα from the resistant cells was more hypophsophorylated compared to the parental cells as well as loss of phosphorylation sites located in the c-terminal domain. other sources have seen this same trend and have reported hyperphosphorylation of topii in etoposide-resistant cells and that the topiiα located in these etoposide-resistant cells have a mutation at the amino acid residues ser861-phe. catalytic inhibitors catalytic inhibitors are the other main identification of topii inhibitors. common catalytic inhibitors are bisdioxopiperazine compounds and sometimes act competitively against topii poisons. they function to target enzymes inside the cell thus inhibiting genetic processes such as dna replication, and chromosome dynamics. additionally, catalytic poisons can interfere with atpase and dna strand passageways leading to stabilization of the dna intermediate covalent complex. because of these unique functions, research has suggested that bis(2,6-dioxopiperazines) could potentially solve issues with cardiac toxicity caused by anti-tumor antibiotics. furthermore, in preclinical and clinical settings, bis(2,6-dioxopiperazines) is used to reduce the side effects of topii poisons. common catalytic inhibitors that target topii are dexrazoxane, novobiocin, merbarone and anthrycycline aclarubicin. dexrazoxane novobiocin merbarone anthrycycline aclarubicin dexrazoxane also known as icrf-187 is currently the only clinically approved drug used in cancer patients to target and prevent anthrycycline mediated cardiotoxicity as well as prevent tissue injuries post extravasation of anthrocyclines. dexrazoxane functions to inhibit topii and its effects on iron homeostasis regulation. dexrazoxane is a bisdioxopiperazine with iron-chelating, chemoprotective, cardioprotective, and antineoplastic activities. novobiocin is also known as cathomycin, albamycin or streptonivicin and is an aminocoumarin antibiotic compound that functions to bind to dna gyrase and inhibits atpase activity. it acts as a competitive inhibitor and specifically inhibits hsp90 and topii. novobiocin has been investigated and used in metastatic breast cancer clinical trials, non-small lung cancer cells and treatments for psoriasis when combined with nalidixic acid. additionally, it is regularly used as a treatment for infections by gram-positive bacteria. novobiocin is derived from coumarin and the structure of novobiocin is similar to that of coumarin. synthetic lethality with deficient wrn expression synthetic lethality arises when a combination of deficiencies in the expression of two or more genes leads to cell death, whereas a deficiency in expression of only one of these genes does not. the deficiencies can arise through mutations, epigenetic alterations or inhibitors of the genes. synthetic lethality with the topoisomerase inhibitor irinotecan appears to occur when given to cancer patients with deficient expression of the dna repair gene wrn. the analysis of 630 human primary tumors in 11 tissues shows that hypermethylation of the wrn cpg island promoter (with loss of expression of wrn protein) is a common event in tumorigenesis. wrn is repressed in about 38% of colorectal cancers and non-small-cell lung carcinomas and in about 20% or so of stomach cancers, prostate cancers, breast cancers, non-hodgkin lymphomas and chondrosarcomas, plus at significant levels in the other cancers evaluated. the wrn protein helicase is important in homologous recombinational dna repair and also has roles in non-homologous end joining dna repair and base excision dna repair. a 2006 retrospective study, with long clinical follow-up, was made of colon cancer patients treated with the topoisomerase inhibitor irinotecan. in this study, 45 patients had hypermethylated