Anti-pathogenic drugs

Question 1

Antibacterial mechanism of action

Answer 1

• Differences between bacteria and mammalian cells: peptidoglycan wall (unique to prokaryotes), plasma membrane has no sterols (eukaryotes), has a single, circular chromosome, has very specific protein synthesis, and energy metabolism differs (no mitochondria)
• Class I reactions utilise glucose and nutrients from environment to produce precursor molecules and energy, class II reactions lead to production of hexoamines, amino acids, and nucleotides, and class III form collection of chemical reactions that produce large bacterial specific molecules
• Class I reactions are similar to bacterial reactions so not a good target, but class II can be used as targets it has some reactions specific to bacteria:
• Folic acid is essential for growth of bacteria (crucial for nucleic acid synthesis) where many bacteria synthesise their own folic acid from PABA
• Sulphonamides are structural analogues of PABA so enters sequence in place of PABA and compete for enzyme to create non-functional analogue of folic acid (can also use trimethoprim) that is no use to bacteria so growth ceases (sulphonamides = bacterial static)
• Best targets are class III reactions as every bacterial cell has to make its own specific macromolecules, so interrupting this process enables stop of bacteria proliferation and can kill bacteria:
• Antibacterial effects can also been seen through increasing cytoplasmic membrane permeability which increases bacteria membrane exposure to environment
• Polymyxin B is effective against gram negative bacteria whereas gramicidin is effective against gram positive
• Targeting protein synthesis in bacteria: streptomycin interferes with anticodon recognition so triple code will be misread by bacterial ribosome leading to no active proteins being formed or erythromycin inhibits translocation of tRNA from A side to P side leading to production of short polypeptides
• Drugs that interfere with energy metabolism: metronidazole is effective against anaerobic bacteria and protozoa have cytotoxic products that destroys cells

Question 2

Anti-viral drug strategies

Answer 2

A. Target virus outside host cell:
1. Vaccine
- Substance that induces immune response, for herd immunity, 60-70% of population needs to be vaccinated, waning can occur when vaccine requires a booster to longer lasting effects
2. Neuraminidase inhibitors
- Acts as a competitive inhibitor for activity of viral neuraminidase upon sialic acid (found on glycoproteins on surface of host cells), blocking activity of enzyme prevents viral particle release through cleaving the sialic acid on glycosylated hemagglutinin so fail to facilitate virus release
B. Inhibition of viral nucleic acid replication
1. DNA polymerase inhibition
- Acyclovir is converted to acyclovir triphosphate which competitively inhibits viral DNA polymerase, incorporates into and terminates growing viral DNA chain so inactivates viral DNA polymerase
2. Reverse transcriptase inhibitors
- Nucleoside reverse transcriptase inhibitors are analogues of the natural nucleosides involved in DNA transcription of virus, NRTIs are active by phosphorylation, competitively bind to enzyme to terminate further DNA chain formation (lamivudine)
- NNRTIs bind at hydrophobic site remote from active site to cause conformational change to prevent substrate binding (efavirenz)
3. RNA-dependent RNA polymerase inhibitors
- RdRp is an important target, it plays a role in replication of RNA genome, host lacks a functional equivalent to this protein
C. Inhibition of viral nucleic acid integration
- Raltegravir: act by inhibiting HIV DNA integrase (enzyme splices viral DNA into host genome when forming the provirus)
D. Target viral protein manufacture
- Specific protease inhibits binding to the site where cleavage occurs (lopinavir + ritonavir)

Question 3

Anti-parasitic drugs

Answer 3

• Parasites avoid immune system by hiding in liver/erythrocyte tissue, host defends by developing cytotoxic CD8+T cells and T helper pathway cytokines (interleukins/ tumour necrosis factor/ interferon-y), activate macrophages to kill infected cells + intracellular parasite
• Sporozoites (asexual parasite form) introduced in host following bite, develop into:
  1. Schizonts = liberate merozoites and infect RBCs, form motile trophozoites, after development release another batch of erythrocyte-infecting merozoites → fever
  2. Dormant hypnozoites = may liberate merozoites in later exoerythrocytic stage
  3. Merozoites can develop into gametocytes (sexual form), they give rise to further stages of parasitic life cycle within insect
• P. vivax = tertian malaria, P. falciparum = malignant tertian malaria (plasmodium has no exoerythrocytic stage)
• Life cycle of parasite: sexual cycle (in anopheles) and asexual cycle (in humans)
• Infection by bite introduces parasite into blood → enters pre/exoerythrocytic cycle in liver and erythrocytic cycle in blood → from blood, sporozoite enters liver cells
• Schizont develops in liver cells → ruptures to release merozoites → enter in RBCs → form motile trophozoites, following division + multiplication, schizonts develop in RBCs → rupture to release further merozoites (parasitize other RBCs)
• Merozoites can develop into male/female gametocytes = fresh source of infective material if blood is consumed by another mosquito

Question 4

Antimalarial drugs

Answer 4

1. Drugs used to treat acute attack
   - Blood schinzoticidal agents used to produce a ‘suppressive’ response for P. vivax or P. ovale, but ‘clinical cure’ for P. falciparum or P. malariae (o exoerythrocytic stage)
   - Drugs: artemisinin and related derived compounds, quinoline-methanols and various 4-aminoquinolines, agents that interfere with synthesis of folate or its action, and atovaquone
   - Combinations are used: quinolones inhibit haem polymerase
2. Drugs that effect a radical cure
   - Tissue schizonticidal agents eradicate P. vivax and P. ovale parasites in liver, only 8-aminoquinolines has this action, they destroy gametocytes → reduced spread of infection
3. Drugs for chemoprophylaxis (prophylactic drugs)
   - Block link between exoerythrocytic stage and erythrocytic stage → prevents development of malarial attacks
   - True casual prophylaxis = prevention of infection by killing sporozoites on entry into host (not feasible with present drugs)
   - Clinical attacks prevented by chemoprophylactic drugs → kill parasite when emerge from liver after exoerythrocytic stage, includes: artemisinin derivates, chloroquine, lumefantrine (also used in combination)
4. Drugs used to prevent transmission
   - Destroy gametocytes, prevent transmission, diminish human reservoir of disease
5. Drug resistance
   - Resistance due to appearance of spontaneously arising point mutation, e.g in target proteins - dihydrofolate reductase
   - Mutations in parasite transporter that facilitate entry/control exit of quinolone drugs into digestive vacuoles can also confer resistance
   - Increase in multidrug resistance may be due to poor compliance, poor drugs, or local variations in host immune responses to infection

Question 5

Types of anti-cancer drugs

Answer 5

• Cell cycle specific = must be in cell cycle, inhibits cell growth at specific phases, cell cycle non-specific = acts on all cells independent of if they are cycling
  1. Alkylating agents, e.g cyclophosphamide and chlorambucil
• Covalently transfer alkyl groups to DNA bases, mono-alkylation → single strand DNA breaks, alkylation of 2 bases → cross bridges prevent DNA being separated
• Pharmacological actions: cytotoxic effects, interfere with DNA integrity/induction of apoptosis
• p53 in non-dividing cells: apoptosis not induced, mutant p53 → resistance to alkylating agents, mutant p53 proteins lose tumour suppressive activities + gain additional oncogenic functions (cells have growth and survival advantage)
• p53 mutation contributed to tumour initiation/promotion/aggressiveness/metastasis
  2. Platinum agents
• Covalently bind to purine DNA bases, bifunctional intra strand cross links, prevent DNA double strand separation, not S-phase specific, binding of cisplatin to DNA = irreversible + structurally different adducts formed
• Adducts classified as intra-strand crosslinking of 2 nucleobases of single DNA strands, inter-strand crosslinking of 2 different strands of 1 molecules = chelate formation by N and O atoms of guanine and DNA-protein crosslinks
  3. Anti-metabolites
• DNA synthesis, cell in most active state, cell specific (S-phase) interfere with incorporation of nucleic acids
• Purine/pyrimidine analogues, pyrimidine antimetabolite = 5FU and capecitabine
• 5 fluorouracil = structural thymidine analogue, binds to thymidine synthase → enzyme inhibited conversion of deoxy uridine to thymidine nucleotides fails → reduced DNA synthesis = decreased proliferation
  4. Topoisomerase inhibitors (irinotecan and camptothecin)
• Topo1 relaxes DNA supercoiling, generates DNA single-strand breaks allow rotation of cleaved strand around double helix axis, re-ligates cleaved strand = re-establish intact duplex DNA
• Topo1 inhibitors stabilise cleavable complexes, prevent DNA re-ligation and induce lethal DNA strand breaks, Topo2 inhibitors form complex with Topo2 after cleavage to inhibit re-ligation and leaves single/double strand breaks
  5. Microtubule inhibitors
• Destabilising microtubules high affinity, bind to ends, splaying and spiralling low affinity, bind to sides, function in metaphase arrest, cause toxicity in neuro/GI/marrow systems
• Stabilising microtubules inhibit dynamic reorganisation, taxanes block cell progression by centrosomal impairment, induction of abnormal spindles and suppression of microtubule dynamics
• Triggering apoptosis by aberrant mitosis. subsequent multinucleated G1-like state related to mitotic-slippage depends on cell type + drug schedule

Question 6

Combination therapy and immune checkpoint inhibitors

Answer 6

• Tumour growth ≠ proceeds exponentially, proportion of non-dividing cells and growth fraction may vary as a function of tumour size
• Each dose ≠ same proportional log kill, proportional kill may relate to growth fraction, tumours = heterogenous, large tumours likelier have drug resistant clones)
• Intensity ≠ influence outcome, cytotoxic drugs given close to max tolerated dose
• Size of growth fraction ↓ exponentially with exponential tumour growth, size of growth fraction max = tumour is 37% of max size
• Combination chemotherapy aims: achieve maximum cell kill with tolerable toxicity, increase kill fraction of resistance cell in heterogenous tumour, and prevent/slow outgrowth of resistant malignant clones
• Only drugs shown to be partially effective used to achieve complete response, avoid overlapping of toxicities on single organ system, there is optimal dose and schedule, treatment-free interval should be shortest compatible with recovery of most sensitive normal tissue, and sequential regimens outperform alternating regimens
• Pharmacogenomics = personal therapy where chemotherapy is tailored around a person’s genetics
• Immune checkpoint = prevent immune response from being strong that it destroys healthy cells
• Inhibitors work by blocking checkpoint proteins from binding to partner to prevent ‘off’ signal, they reinvigorate antitumor immune response by interrupting co-inhibitory signalling pathways and promotes immune-mediated elimination of tumour cells (blocks CTLA4, PD1 and PD-L1)
• Checkpoints engage when surface proteins of immune cell recognise and bind to partner proteins on other cell, to send an ‘off’ signal to T cell