Curaxins is a family of carbazole-based small molecules with anti-cancer activity include modulation of the effects of multiple transcription factors, several DNA-related enzymes and the histone chaperone FACT. All of these effects appear to stem from the ability of curaxins to bind genomic DNAin cells with high affinity, but without induction of DNA damage. Although curaxin binding does not induce chemical DNA modifications, it does alter the shape, flexibility and electrical charge of the DNA helix. These changes make DNA less capable of wrapping around histones to form nucleosomes, the basic units of chromatin. This leads to the loss of histones from chromatin in curaxin-treated cells and numerous downstream effects on gene expression and DNA metabolism that impact cell growth and survival. Thus, analogous to DNA damaging drugs, curaxins have been classified as “chromatin damaging” agents. In fact, induction of chromatin damage in the absence of DNA damage was first proposed as a promising anti-cancer approach based on the mechanism of action and biological activity of curaxins.
Mechanism of action
Current understanding of the mechanism of action of curaxins is that the biological effects of these compounds stem from their ability to bind genomic DNA in cells, which results in chromatin disassembly but not chemical DNA damage. DNA is a flexible double helical polymer that has certain parameters under physiological conditions in cells that are critical for its biological function; these include its shape (length, width, degree of twisting), flexibility, and negative electrical charge. Curaxins binding to DNA does not cause any detectable chemical DNA modifications (i.e., breaks or nucleotide alterations). However, it does change the shape of the DNA molecule (increases its length due to intercalation of curaxin molecules between base pairs), reduce its flexibility, and diminish its negative charge (since curaxins are positively charged). While DNA can tolerate all of these changes without undergoing any disintegration, the basic units of chromatin, nucleosomes, cannot.
A nucleosome consists of a positively charged central protein octamer around which 146-147 base pairs of genomic DNA is wrapped ~1.7 times. The exceptional stability of nucleosomes is provided by (I) electrostatic attraction between positively charged histone proteins and negatively charged DNA, (II) multiple point contacts between amino acids of histone proteins and DNA, with specific amino acid side chains intercalating between DNA base pairs or filling the minor groove, and (III) the perfect fit of the DNA helix around the histone core based on the flexibility of DNA and the diameter of the histone octamer. Curaxin binding to DNA disturbs each of these aspects of nucleosome stability, reducing the negative charge of DNA, interfering with contacts between histone amino acids and DNA, and making DNA more rigid and less able to wrap around histone octamers tightly. Indeed, curaxin-induced nucleosome destabilization and disassembly has been observed both in cells and under cell-free conditions, leading curaxins to be defined as chromatin-damaging agents.
Curaxins cause the following effects in cells:
C-trapping of FACT
FACT (Facilitates Chromatin Transcription) is a histone chaperone that is involved in multiple chromatin-related processes, such as transcription, replication and DNA repair. It binds components of nucleosomes (histone dimers and DNA) via different domains of its two subunits, SSRP1 and SPT16, thereby controlling nucleosome accessibility and stability. In curaxin-treated cells, destabilization of nucleosomes exposes multiple sites for FACT binding that are normally hidden within assembled nucleosomes. This leads to FACT becoming tightly bound in chromatin (referred to as chromatin-trapping, or c-trapping) and depleted from regions of active transcription, and probably from replication sites, where it is enriched under basal conditions. The redistribution of FACT in curaxin-treated cells thus results in its functional inhibition. Notably, FACT levels are elevated in various types of tumors and correlate with poor prognosis, providing a mechanistic explanation for the observed anti-cancer activity of curaxins.
Activation of tumor suppressor p53
While curaxins were discovered based on their ability to cause p53 activation, exactly how they do this remains unclear. The two most likely mechanisms are: (I) When FACT is trapped in chromatin, FACT-associated casein kinase 2 phosphorylates p53 at serine 392 (no other phosphorylated sites were found in p53 in curaxin-treated cells) and (II) histones lost from chromatin in curaxin-treated cells accumulate in the nucleolus and this organelle is known to contain proteins that bind and inactivate MDM2, a major negative regulator of p53, upon nucleolar disintegration (“nucleolar stress”). Therefore, accumulation of histones in nucleoli may be responsible for nucleolar stress and inhibition of MDM2 leading to stabilization of p53. While p53 in curaxin-treated cells lacks the post-translational modifications characteristic of DNA damage-dependent activation, it is fully functional and capable of transactivation and induction of growth arrest or apoptosis.
Induction of type I interferon response
The type I interferon response is a physiological immune response in mammals that is initiated upon detection of products of viral metabolism (e.g., double stranded RNA) by specific intracellular receptors. Receptor activation leads to synthesis of interferons alpha and beta, which in turn bind their own receptor and stimulate expression of interferon stimulated genes (ISGs) encoding proteins with roles in blocking viral expansion or eliminating infected cells. Destabilization of nucleosomes by curaxins leads to increased accessibility of heterochromatin and desilencing of the various types of repetitive elements it contains (e.g., centromeric or pericentromeric repeats, endogenous retroviruses, etc.). Moreover, transcription of these elements occurs in both directions, thus leading to the emergence of double stranded RNAs capable of inducing a type I interferon response.
Dysregulation of transcription
Curaxin-induced changes in chromatin structure have profound effects on transcription. Some genes, such as those regulated by p53 or ISGs become activated, while many others are inhibited (e.g., MYC and genes regulated by NF-kappaB, HSF1, etc.). While upregulation of p53-regulated genes and ISGs makes sense due to the observed stabilization of p53 and production of interferons in curaxin-treated cells, the mechanisms underlying inhibition of expression of other genes are not completely clear. Possible explanations include disruption of promoter/enhancer interactions or occurrence of pervasive transcription following curaxin binding to DNA. Loss of nucleosomes is a known trigger of transcription initiated from cryptic promoters that are not utilized under basal conditions. While the origins of these cryptic promoters are not completely clear, some of them are remnants of ancient retroviral LTRs, that are kept silent by being packaged in heterochromatin. Relaxation of the normal transcriptional controls imposed by chromatin structure results in pervasive, cryptic, and divergent (i.e., proceeding in both sense and anti-sense directions) transcription, all of which may interfere with normal transcription.
Cellular growth arrest
Growth arrest is observed in cells treated with the relatively low concentrations of curaxins that do not induce significant nucleosome disassembly, but only chromatin opening accompanied by eviction of linker histone H1 and occasional loss of outer histones H2A and H2B. This effect is reversible upon drug removal, especially in normal cells. In tumor cells, continued exposure to curaxins results in transition from growth arrest to cell death.
Cell death is caused by the higher concentrations of curaxins that induce eviction of core histones from chromatin. If cells express wild type p53, curaxin-induced cell death is accompanied by activation of caspases in the cytoplasm similarly to apoptotic cell death. However, the nuclear aspects of cell death induced by curaxins are different from those observed in apoptosis: there is no characteristic chromatin condensation or nuclear fragmentation. This may be due to inhibition of caspase-activated nucleases, which are responsible for DNA cleavage in the case of apoptosis, by curaxins. Cell lacking functional p53 die following curaxin exposure via a caspase-independent mechanism that has not yet been fully studied.
The clinical lead curaxin CBL0137, demonstrated significant anti-cancer activity in a number of preclinical tumor models, including colon, pancreatic, prostate and breast adenocarcinomas, melanoma, kidney cancer, glioblastoma, hematological malignancies, and several pediatric malignancies, including neuroblastoma. It showed efficacy both as a monotherapy and in combination with standard of care and various targeted therapies. In animal studies, significant inhibition of tumor growth, and in some cases complete tumor eradication, was observed at non-toxic concentrations of. CBL0137 is currently being tested in several clinical studies against solid and hematological malignancies. (View Clinical Trials)
Curent information on CBL0137 research can be found in the publications section.