lecture 14 - Bleomycins Flashcards
What is bleomycin?
It is a mixture of cytotoxic glycopeptide antibiotics isolated from a bacterium and is freely soluble in water
They are a related group of glycopeptides which differ in the terminal amine substituent of the common structural unit, Bleomycin acid
they are usually used clinically in combination chemotherapy against lymphomas, squamous cell carcinomas and germ-cell tumours
Their damage is similar to that generated by ionising radiation
What is the structure of bleomycin (main parts)?
It contains a metal-binding domain, a linker region + bithiazole tail, and a disaccharide
The bithiazole tail slots through the DNA
How do bleomycins induce dsDNA cleavage and what is the proposed model for this process?
Bleomycins initiate ssDNA cleavage - occurs at pyrimidines 3’ to a guanine
A model for dsDNA cleavage by a single bleomycin reactivation and bleomycin reorganisation during or after cleavage of the first strand of DNA:
The key to the reorganisation is the linker and the flexibility of the bithiazole tail that is bound by partial intercalation
the rotation around the bond between the two thiazole rings in the tail of bleomycin, together with other motions, make the peroxide of the activated drug available for interaction with the second DNA strand
What are the most common cellular outcomes of bleomycin treatment?
Extended cell-cycle arrest, apoptosis and mitotic death
What is the main way that DNA DSBs can be repaired?
Recombination is the only error-free pathway to accomplish this - involves steps that are only now beginning to be understood
Non-homologous end joining (NHEJ) and break-induced replication (BIR) are error-prone, leading to mutations and loss of heterology
What is the general mechanism of action of platinum anti-cancer drugs?
They have the ability to crosslink with the purine bases on the DNA to form DNA adducts, preventing repair of the DNA leading to DNA damage and subsequently induces apoptosis within cancer cells
Cisplatin binds to the N7 reactive center on purine residues and as such can cause DNA damage in cancer cells, blocking cell division and resulting in apoptotic cell death. The 1,2-intrastrand cross-links of purine bases with cisplatin are the most notable among the changes in DNA
Bends the DNA - makes it difficult to recognise for repair mechanisms
What are some tumour resistance mechanisms to cisplatin and carboplatin (before DNA binding)?
Mediated by inadequate levels of platinum reaching target DNA
Platinum might enter cells using transporters (e.g. CTR1) or by passive diffusion
Loss of CRT1 results in less platinum entering cells, and consequently, drug resistance
Once inside cells, cisplatin is activated by the addition of water molecules to form a chemically reactive aqua species - this is facilitated by relatively low chloride concentrations found within the cells
In the cytoplasm, the activated aqua species react with species containing high sulphur levels, such as glutathione
In some platinum-resistant cancer cells, glutathione levels are high, so the platinum is “mopped up” before DNA binding can occur, causing resistance
Active export of platinum from the cells through copper exporters can also contribute to platinum drug resistance
Cancer could reduce expression of copper pumps - if down regulated, less gets into the cell
How can cancer become resistant to cisplatin/carboplatin after DNA binding?
Once the activated aqua platinum species has entered the nucleus, preferential covalent binding to the nitrogen on position 7 of guanine occurs
The major covalent bis-adduct that is formed involves adjacent guanines on the same strand of DNA - a minor adduct involves binding to guanines on opposite DNA strands
The main removal pathway for these DNA adducts is that of NER - increased NER, especially through increased activity of endonuclease ERCC1, can occur in tumours and lead to platinum drug resistance (as adducts are removed before apoptotic signalling pathways are triggered)
Resistance can also occur through increased tolerance to platinum-DNA adducts (even though the DNA adducts are formed), either through: loss of DNA mismatch repair, bypassing of adducts by polymerase b and n, or through down regulation of apoptotic pathways
What are some ongoing strategies to circumvent cisplatin and carboplatin resistance?
Increasing the levels of platinum reaching tumours (e.g. liposomal platinum products), thereby resulting in greater killing
Combining existing platinum drugs with molecularly targeted drugs (e.g. bevacizumab)
Using novel platinum drugs that are capable of circumventing cisplatin-mediated resistance mechanisms
Using drugs (alone or in combination) which exploit particular cisplatin-mediated resistance mechanisms
