Antineoplastic resistance

Antineoplastic resistance, often used interchangeably with chemotherapy resistance, is the multiple drug resistance of neoplastic (cancerous) cells, or the ability of cancer cells to survive and grow despite anti-cancer therapies.

There are two general causes of antineoplastic therapy failure:^[1] Inherent properties, such as genetic characteristics, giving cancer cells their resistance,^[1] which is rooted in the concept of cancer cell heterogeneity and acquired resistance after drug exposure.^[1] Cancer cells can become resistant to multiple drugs by various mechanisms, including:^[1][2] Altered membrane transport, enhanced DNA repair, apoptotic pathway defects, alteration of target molecules, protein and pathway mechanisms, such as enzymatic deactivation. Since cancer is a genetic disease,^[3] two genomic events underlie these mechanisms of acquired drug resistance: Genome alterations (e.g. gene amplification and deletion) and epigenetic modifications. Cancer cells are constantly using a variety of tools, involving genes, proteins and altered pathways, to ensure their survival against antineoplastic drugs. This list of mechanism is in no way exhaustive. New and existing ideas of drug resistance mechanisms are being explored and studied, as well as the complex and challenging ways in overcoming such treatment resistance.

Cancer cell heterogeneity

Cancer cell heterogeneity, or tumour heterogeneity, is the idea that tumours are made up of different populations of cancer cells that are morphologically, phenotypically and functionally different.^[4] Certain populations of cancer cells may possess inherent characteristics, such as genetic mutations and/or epigenetic changes, that confer drug resistance. Administration of antineoplastic drugs kills non-resistant sub-populations and favors those resistant cancer cells. While the tumour mass may shrink as an initial response to the drug, resistant colonies will survive treatment, be selected and then propagate, eventually causing a cancer relapse.

A slightly different way in which cancer cell heterogeneity can give rise to disease progression is via targeted therapy, a type of treatment that targets a specific molecular marker. Tumour cells that do not express the specific marker are not killed, and are then able to divide and mutate further, creating a new heterogeneous tumour, which complicates antineoplastic treatment.

Mechanisms of acquired resistance

An overview of antineoplastic resistance mechanisms, and examples of the major genes involved.

Alteration of membrane transport

Many classes of antineoplastic drugs acts on DNA, nuclear components, and intracellular pathways, meaning that they require entry into the cancer cells. The p-glycoprotein (P-gp), or the multiple drug resistance protein, is a phosphorylated and glycosylated membrane transporter that can shuttle drugs out of the cell, thereby decreasing or ablating drug efficacy. This transporter protein is encoded by the MDR1 gene and is also called the ATP-binding cassette (ABC) protein. MDR1 has promiscuous substrate specificity, allowing it to be able to transport many structurally diverse compounds across the cell membrane, mainly hydrophobic compounds. Studies have found that the MDR1 gene can be activated and overexpressed in response to drug administration, thus forming the basis for resistance to many drugs.^[1] Overexpression of the MDR1 gene in cancer cells is used to keep intracellular levels of antineoplastic drugs below cell-killing levels.

For example, rifampicin has been found to induce MDR1 expression. Experiments in different drug resistant cell lines and patient DNA revealed gene rearrangements that had initiated the activation or overexpression of MDR1.^[5] A C3435T polymorphism in exon 226 of MDR1 has also been strongly correlated with p-glycoprotein activities.^[6]

Studies have shown that MDR1 activation occurs through activation of NF-κB, a protein complex that acts as a transcription factor.^[7][8] There is a NF-κB binding site located adjacent to the rat mdr1b gene,^[9] and studies show that MDR1 upregulation is reliant on NF-κB activation as well.^[10][11] NF-κB is active in tumour cells due to mutations in the NF-κB gene or in the inhibitory IκB gene. In colorectal cancer cells, inhibition of NF-κB or MDR1 resulted in increased apoptosis in response to a chemotherapeutic agent used to treat various cancers.^[7]

Enhanced DNA repair

Enhanced DNA repair plays an important role in the ability for cancer cells to overcome drug-induced DNA damages.

Platinum-based chemotherapies, such as cisplatin, target tumour cells by cross-linking their DNA strands, causing mutation and damage.^[1] Such damage will trigger programmed cell death (e.g. apoptosis) in cancer cells. Cisplatin resistance occurs when cancer cells develop an enhanced ability to reverse such damage, by removing the cisplatin from DNA and repairing any damage done.^[1][2] The upregulated expression of the excision repair cross-complementing (ERCC1) gene and protein in cisplatin-resistant tumors underlies this process.^[1]

Some chemotherapies are alkylating agents that result in the attachment of an alkyl group to DNA. O6-methylguanine DNA methyltransferase (MGMT) is a DNA repair enzyme that removes alkyl groups from DNA. Expression of MGMT is upregulated in many cancer cells, providing a protective function from alkylating agents.^[2] Increased MGMT expression has been found in colon cancer, lung cancer, non-Hodgkin’s lymphoma, breast cancer, gliomas, myeloma and pancreatic cancer.^[12]

Apoptotic pathway defects

TP53 is a tumor suppressor gene that encodes the p53 protein. It is a protein that responds to DNA damage, either by initiating DNA repair, cell cycle arrest, or apoptosis. The loss of TP53 via gene deletion can result in continued replication despite DNA damage. This tolerance of DNA damage can grant cancer cells a method of resistance to drugs aiming to induce apoptosis via DNA damage.^[1][2]

Other genes involved in the apoptotic pathway related drug resistance includes h-ras and bcl-2/bax.^[13] Oncogenic h-ras has been found to increase expression of ERCC1, resulting in enhanced DNA repair (see above).^[14] Inhibition of h-ras was found to increase cisplatin sensitivity in glioblastoma cells.^[15] Upregulated expression of Bcl-2 in leukemic cells (non-Hodgkin’s lymphoma) resulted in decreased levels of apoptosis in response to chemotherapeutic agents, as Bcl-2 is a pro-survival oncogene.^[16]

Alteration of target molecules

During targeted therapy, oftentimes the target of the therapy is modified and its expression decreased to the point that it is no longer an effective target for therapy. One example of this is the loss of estrogen receptor (ER) and progesterone receptor (PR) upon anti-estrogen treatment of breast cancer.^[17] Patients with loss of ER and PR are no longer receptive to tamoxifen or other anti-estrogen treatments, and while cancer cells remain somewhat responsive to estrogen synthesis inhibitors, they eventually become unresponsive to endocrine manipulation and no longer dependent on estrogen for growth.^[17]

Another line of therapeutics used for treating breast cancer is targeting of kinases like human epidermal growth factor receptor 2 (HER2) from the EGFR family. Mutations often occur in the HER2 gene upon treatment with an inhibitor, with about 50% of patients with lung cancer found to have an EGFR-T790M gatekeeper mutation.^[2]

Treatment of chronic myeloid leukemia (CML) involves a tyrosine kinase inhibitor that targets the BCR/ABL fusion gene called imatinib. Studies have found that in some patients that have become resistant to Imatinib, the BCR/ABL gene is reactivated or amplified, or a single point mutation has occurred on the gene. These point mutations enhance autophosphorylation of the BCR-ABL protein, resulting in the stabilization of the ATP-binding site into its active form, which cannot be bound by imatinib for proper drug activation.^[18]

Topoisomerase is a lucrative target for cancer therapy due to its critical role as an enzyme in DNA replication, and many topoisomerase inhibitors have been made.^[19] Resistance can occur when topoisomerase levels are decreased, or when different isoforms of topoisomerase are differentially distributed within the cell. Mutant enzymes have also been reported in patient leukemic cells, as well as mutations in other cancers that confer resistance to topoisomerase inhibitors.^[19]

Protein and pathway mechanisms

One of the mechanisms of antineoplastic resistance is over-expression of drug-metabolizing enzymes or carrier molecules.^[1] By increasing expression of metabolic enzymes, drugs are more rapidly converted to drug conjugates or inactive forms that can then be excreted. For example, increased expression of glutathione promotes drug resistance, as the electrophilic properties of glutathione allow it to react with cytotoxic agents, inactivating them.^[20] In some cases, decreased expression or loss of expression of drug-metabolising enzymes confers resistance, as the enzymes are needed to process a drug from an inactive form to an active form. Arabinoside, a commonly used chemotherapy for leukemia and lymphomas, is converted into cytosine arabinoside triphosphate by deoxycytidine kinase. Mutation of deoxycytidine kinase or loss of expression results in resistance to arabinoside.^[1] This is a form of enzymatic deactivation.

Growth factor expression levels can also promote resistance to antineoplastic therapies.^[1] In breast cancer, drug resistant cells were found to express high levels of IL-6, while sensitive cells did not express significant levels of the growth factor. IL-6 activates the CCAAT enhancer-binding protein transcription factors which activate MDR1 gene expression (see Alteration of Membrane Transport).^[21]

Genomic mechanisms underlying acquired resistance

Genome alterations

Chromosomal rearrangement due to genome instability can cause gene amplification and deletion, both of which underlie the development of multidrug resistance.

• Gene amplification is the increase in copy number of a region of a chromosome.^[22] Such events occur frequently in solid tumors, and can contribute to tumor evolution through altered gene expression.^[22] Initiation of gene amplification occurs on the basis of genome instability. Research on Chinese hamster cells showed that amplifications in the DHFR gene (involved in DNA synthesis) began with chromosome break in distal areas to the gene, and subsequent cycles of bridge-breakage-fusion formations result in large intrachromosomal repeats.^[23] The over amplification of oncogenes can occur in response to drug administration, and this is thought to be the underlying mechanism in several classes of resistance.^[22] For example, DHFR amplification occurs in response to methotrexate,^[24] TYMS (involved in DNA synthesis) amplification occurs in response to 5-fluorouracil,^[25] and BCR-ABL amplification occurs in response to imatinib mesylate.^[26] Determining areas of gene amplification in patients using sequencing techniques has huge clinical implications.
• Gene deletion is the opposite of gene amplification. It is where a region of a chromosome is lost. Gene deletion confers drug resistance through the loss of tumor suppressor genes such as TP53.^[1]

Epigenetic mechanisms

Epigenetic modifications in antineoplastic drug resistance play a major role in cancer development and drug resistance as they contribute to the regulation of gene expression.^[2] Two main types of epigenetic control are DNA methylation and histone methylation/acetylation. DNA methylation is the process of adding methyl groups to DNA, usually in the upstream promoter regions, which stops DNA transcription at the region and effectively silences individual genes. Histone modifications, such as deacetylation, alters chromatin formation and silence large chromosomal regions. In cancer cells, where normal regulation of gene expression breaks down, the oncogenes are activated via hypomethylation and tumor suppressors are silenced via hypermethylation. Similarly, in drug resistance development, it has been suggested that epigenetic modifications can result in the activation and overexpression of pro-drug resistance genes.^[2]

Studies on cancer cell lines have shown that hypomethylation (loss of methylation) of the MDR1 gene promoter resulted in its overexpression and the acquisition of multidrug resistance.^[27]

In a methotrexate resistant breast cancer cell line lacking drug uptake and folate carrier expression, administration of DAC, a DNA methylation inhibitor, effectively improved drug uptake and folate carrier expression.^[28]

Acquired resistance to the alkylating drug fotemustine in melanoma cell showed high MGMT activity related to the hypermethylation of the MGMT gene exons.^[29]

In Imatinib resistant cell lines, silencing of the SOCS-3 gene via methylation has been shown to cause STAT3 protein activation, which resulted in uncontrolled proliferation.^[30]

Genetic markers for drug sensitivity and resistance

Pharmacogenetics is playing an increasingly important role in antineoplastic treatment.^[31] With the advancing rapid sequencing technologies, identifying and testing genetic markers for treatment sensitivity and potential resistance are becoming more accessible. While many genetic markers are being studied, certain markers are more representative and possess higher potential for clinical applications.^[31]

Characteristics of Pharmacogenetic Tests^[31]

Marker	Drug	Major Conditions	Clinical Implications
TYMS	5-Fluorouracil	Colorectal, stomach, pancreatic cancer	High TYMS may show poor response & less toxicity
DPYD	5-Fluorouracil	Colorectal, stomach, pancreatic cancer	DPD deficiency associated with higher risk of toxicity
UGT1A1	Irinotecan	Colorectal cancer	Decreased UGT1A1 activity may increase risk of toxicity
CYP2D6	Tamoxifen	Breast cancer	Patients with deficient CYP2D6 activity are at greater risks of relapse
EGFR	Anti-EGFR therapy	Colorectal, lung cancer	Activation of EGFR pathways enhances tumor growth, progression, & resistance to therapy
KRAS	Anti-EGFR therapy	Colorectal, lung cancer	KRAS mutation is associated with resistance to anti-EGFR therapy
FCGR3A	Rituximab	Non-Hodgkin's lymphoma	FCRG3A 158Val/Val genotype may be associated with better response
BRCA1/BRCA2	Platinum	Breast, ovarian cancer	BRCA1/2-mutated cancers are more sensitive to DNA damage. Secondary intragenic mutations confer acquired resistance

Genetic approaches to overcome drug resistance

MDR proteins are known to be drug-resistance genes, and are highly expressed in various cancers. Inhibition of the MDR genes could result in sensitization of cells to therapeutics and a decrease in antineoplastic resistance. Reversin 121 (R121) is a high-affinity peptide for MDR, and use of R121 as a treatment for pancreatic cancer cells results in increased chemosensitivity and decreased proliferation.^[32]

Aberrant NF-κB expression is found in many cancers, and NF-κB has been found to be involved in resistance to platinum-based chemotherapies, such as cisplatin. NF-κB inhibition by genistein in various cancer cell lines (prostate, breast, lung and pancreas) showed increased growth inhibition and an increase in chemosensitivity, seen as an increase in apoptosis induced by therapeutic agents.^[33] However, targeting the NF-κB pathway can be difficult, as there can be many off-target and non-specific effects.

Expression of mutated TP53 causes defects in the apoptotic pathway, allowing cancerous cells to avoid death. Re-expression of the wild-type gene in cancer cells in vitro has been shown to inhibit cell proliferation, induce cell cycle arrest and apoptosis.^[34]

In ovarian cancer, the ATP7B gene encodes for a copper efflux transporter, found to be upregulated in cisplatin-resistant cell lines and tumors. Development of antisense deoxynucleotides against ATP7B mRNA and treatment of an ovarian cancer cell line shows that inhibition of ATP7B increases sensitivity of the cells to cisplatin.^[35]

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