Metals in Medicine

Question 1

Sketch the schematic dose-response diagram of an essential element and define theraputic width.

Answer 1

Theraputic width is the concentration range in which positive physiological effects are observed.

Question 2

What considerations must be made in regard to theraputic width?

Define antagonism and synergism.

Answer 2

Considerations include: the compound in the metals delivery, populations variations and interactions between metals/compounds.

Two compounds can interact to promote mutual effects (synergism - solubilisation) or by competing and supressing each others effects (antagonism - displacement, deactivation and chelation)

Question 3

Give the number of essential metals in humans and the types of metals.

Answer 3

4 main group (Na, K, Mg, Ca) and 9 transition metals (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo)

Question 4

Describe the deficiencies of Fe, Zn, Cu and Co.

Answer 4

1. Fe - anaemia results from lack of O₂ from decrease of haemoglobin (Hb) levels. Most abundant TM in humans.
2. Zn - growth retardation, skin lesions, affects the key enzymes of alkaline phosphatase and carboxy peptidase. 2nd most abundant TM in body.
3. Cu - anaemia, brain and heart disease, has very important role in respiration (cyctochromes) and superoxide deactivation.
4. Co - pernicious anaemia, not enough red blood cells, loss of vitamin B₁₂.

Question 5

Describe how non-bioessential inorganic elements interact with the body.

Answer 5

Some elements are extrememly low abundance and have not been recognised as relevent to life (Bi, Al, lanthanides).

Other elements have exclusively negative effects to the body (Pb, Cd, Hg, Tl). These are toxic for one of two reasons. Soft metals have a very high affinity for SH groups. Substitution of essential metals can occur and replace functionality. (Zn - Cd, Ca - Pb, Cd).

Question 6

How can toxic metal poisoning be treated?

Answer 6

1. Antagonists can be administered to outcompete toxic metals (extra Ca in Cd poisoning).
2. Chelation therapy (such as EDTA). Affinity and selectivity must be carefully designed.

Question 7

Why is chelation favourable?

Answer 7

1. Thermodynamic stability - entropic increase and solvent interactions.
2. Kinetic stability - many donors in close proximity increase ‘holding power’.

Question 8

Looking at the given chelators, describe the donor atoms and other functional groups, and hence give metals that they will chelate.

Answer 8

• Dimercaptol - 2xS coordinators, solubilising OH: binds soft metals like Hg²⁺, Cd²⁺, As³⁺, Ni²⁺
• D-penicillamine - S, N coordinators solubilising COOH: binds soft/intermediate metals like Hg²⁺, Cd²⁺, Pb²⁺, Cu²⁺
• EDTA - 4xO, 2xN coordinators: binds intermediate metals like Ca²⁺, Pb²⁺
• Desferal - 6xO coordinators, many OH, N and CO solubilising groups: binds hard metals like Fe³⁺, Al³⁺

Question 9

Describe in detail how the structure of D-penicillamine allows for its chelation of toxic metals.

Answer 9

• The N and S atoms coordinate to soft metal centres.
• A five membered ring forms which is very stable.
• The chelation effect is thermodynamically favourable.
• The COOH group aids solubility.

Question 10

Describe the symptoms of lead poisoning and how these symptoms arise. What is the retention time of lead and how is it treated?

Answer 10

Symptoms depend on the form of lead - organometallic lead (PbEt₄) causes a disordered nervous system and can cross the blood-brain barrier causing a loss of coordination.

Pb²⁺ salts causes anaemic symptoms as it inhibits a Zn enzyme involved in heme biosynthesis. They also inhibit Fe incorperation into heme.

The retention time in blood, liver and kidneys is 1 month, but up to 30 years in bones.

Treatment is chelation therapy (BAL and EDTA) but chronic poisoning is almost impossible to treat.

Question 11

What are the natural detoxification methods for mercury poisoning?

Answer 11

• Efflux pumps to remove the Hg or to introduce S^2- groups to bind to the ions.
• Using cysteine-rich proteins to chelate the ions, such as metallothioneins.

Question 12

Describe the symptoms and treatment of thallium poisoning.

Answer 12

Substance is colourless, tasteless and odourless. Symptoms include nausea, vomiting, headaches, dementia and hair loss.

Treatment involves ion exchange absorbents which are lattices of Fe(CN)₆ which contain K⁺. The K is exchanged for the Tl⁺ ions.

Question 13

Describe two genetic diseases that can lead to an iron overload in the body. Why does excess iron cause problems?

Answer 13

Sickle cell disease is a single amino acid mutation that causes lower binding forms of Hb.

Thalassaemia is a disease that causes an unbalance production of α and β chains.

Boths these conditions require regular blood transfusions to treat, which causes a huge excess of iron in the body. Normally there is around 4.5 g of iron, but patients with these conditions may accumulate up to 70 g!

Excess iron generates free radicals which attack organic molecules indiscriminately.

Question 14

Desferal is a siderophore mimic, describe how it is a suitable binder of iron and its disadvantages.

Why can’t enterobactin be used?

Answer 14

Desferal has a low toxicity, is very selective for Fe(III), has high water solubility, suitable M_W (>300, <700), is kinetically labile and is charged so doesn’t cross membranes.

However it is not orally active (low complience for intravenous use), must be given over many hour long periods multiple times a week, has a long, expensive synthesis and can have large variations in batch quality.

Enterobactin is not orally active and has pH sensitive groups. It would also contribute to bacteria iron collection.

Question 15

What key functional group has been designed to make water soluble iron binders? Give its advantages and how its properties have been improved.

Answer 15

3-Hydroxypyridin-4-ones (3,4-HPO’s) are isoelectronic and isostructural with catechols (siderphore ligands), and are neutral so can cross cell membranes. They are easily synthesised and orally active.

With R₁ = R₂ = Me, there is a small amount of ML₂ generated at pH 7 which generates radicals. Changing R₂ into CH₂CH₂NHCH₃ improves the properties.

Question 16

Describe the challenges with ²³⁹Pu poisioning and how it can be treated.

Answer 16

²³⁹Pu(III/IV) has a very similar ionic radius to Fe(II/III). This means chelation therapy becomes difficult. Specifity is gained by using an 8 donor chelation agent based on an amide backbone with N-hydroxypiridinone binders.

Question 17

How are metal ions related to neurodegenerative diseases such as Alzheimers, Parkinsons and prion disease?

Answer 17

Common features of all three diseases are changes in the metal homeostasis of the brain. Zn, Fe and Cu are all required for brain function. This change in homeostasis can lead to protein misfolding and formation of radical species.

Question 18

Describe the key features of metal chelators in the treatment of neurodegenerative diseases.

Answer 18

They must be M_W<300 to cross the blood-brain barrier. DFP was been designed to be glycosylated by the transport molecule, GLUT1. DFP has chelatin and anti-oxidant properties.

Question 19

How can resistance to antibiotics be overcome with the use of siderophores?

What are the requirements of this approach?

Answer 19

Siderophores are taken up by bacteria by recognising the molecule and transporting it across the membranes. If this can be linked to a drug, the drug can be delivered directly into a bacteria (Trojan horse).

This has been evolved by streptomycin to kill bacteria who take up the siderophores it releases.

These drugs have to conjugate to the siderophore backbone so it can be release when it breaks down. It must also be small and uncharged.

Question 20

Describe the mode of action of cis-platin and the disadvantages of the drug.

Answer 20

Cis-platin crosslinks DNA by reacting with 2 different nucleophiles, distorting the helix. The drug is taken up by diffusion and hydrolysed to [Pt(NH₃)₂Cl(OH₂)]⁺, the negative charge DNA attracts the species and then replaces the water ligand, typically with a guanine N7.

A high mobility group (HMG) binds to the adducts and blocks the repair of the DNA. This damage stops replication and causes cell death.

Disadvantages of cis-platin is that it has to be adminsitered intravenously and that it has major kidney and gastrointestinal toxicity.

Question 21

Describe how the second generation analogues of cis-platin improved its drug properties.

Answer 21

Second generation:

• All have didentate ligands in place of the Cl ligands so they are more intert. This reduces toxicity, especially the nephrotoxicity (kidney).
• Examples include carboplatin, oxaliplatin and nedaplatin.
• Overall, side effects are reduced and drugs are still active.

Question 22

How is resistance developed for first and second generations of cis-platin?

How can this be addressed?

Answer 22

Glutathione can react with Pt centre through a S group. This makes the drug inactive.

Third generation analogues of cis-platin have a bulky amine ligand (1-3 aromatic rings) which blocks the attack of the glutathione nucleophile.

Question 23

Describe the strengths of the third generation cis-platin analogues based on Pt(IV) and Ru(II/III).

Answer 23

Pt(IV) complexes are more inert (low spin d⁶), only in the cell is the metal reduced to Pt(II). Pt(IV) is octahedral so two additional ligands can be used to tune the drug properties. This allows increased solubility of drugs (enabling chemo for outpatients) and targeting of specific cancer cells (using hormone prodrugs).

Ru(II) complexes have a different mode of action, but similar kinetics to Pt(II). Ru(III) is low toxicity and is reduced to Ru(II) in hypoxic (low O₂) conditions (tumors).

Question 24

Describe how unstable Au(I) can be used to treat rheumatoid arthritis.

Answer 24

Au(I) will typically undergo disproportionation, but can be stabilised by soft ligands.

It treats arthritis by binding to thiols with high affinity. It inhibits thioredoxin reductase which is involved in inflammation. The drug collects at the joints to treat inflammation with specifity.

It can also be used to treat other diseases that depend strongly on thioredoxin reductase, including tropical diseases.

Question 25

How do MRI positive contrast agents work?

Answer 25

Paramagnetic agents give bright MRI images as the water signal intensity depends on proton relaxation rates. The paramagnetic centre increases relaxation rate so the water near the complex give a higher response. This effect is observed with contrast agents in the nM concentration.

Question 26

How can Gd(III) complexes be used as MRI positive contrast agents? What issues have to be addressed?

Answer 26

Gd(III) is f⁷, s = 7/2 so has a large magnetic moment and will rapidally exchange H₂O ligands in solution. This makes it a very good contrast agent. However Gd(III) is highly toxic so it must be carefully chelated. Chelation is done by octadentate ligands which leave a single coordination state for water ligands to exchange at. It is a hard metal so all donors are O or N.

Question 27

The 4 chelators show are all used to chelate Gd(III). Compare their structures and how they can be used for contrast complexes.

Answer 27

All have donor atoms of either O or N for hard binding. Omniscan has one less O and one more N atom which will reduce binding. DOTA is 1000x better at binding than DTPA as it is cyclic which gives the structure additional stability. The bottom two structures form neutral complexes with Gd(III). When patients have kidney disease the complexes are more likely to be released so have to be extremely stable. DTPA and Omniscan have been suspended for sue for this reason. Negatively charged compounds are painful to inject so despite the lower binding ability of Prohance, it is the favoured ligand

Question 28

In what two ways can Gd(III) binders be derivitised?

Answer 28

By carefully designing the amine group to bind to a large protein, it has an even larger decrease in tumbling rate. This leads to greater signal. Other amine groups can be designed to bind to zinc atoms. This again increases the size of the complex to reduce tumbling, and can be used to moniter zinc levels in the blood.

Question 29

What two ways can radiopharmaceuticals be used in medicine? What emitters and half lives are appropriate for each type.

Answer 29

Radionuclide therapy: α and β emitters with half lives of hours to days. Diagnosis: β and γ emitters with half lives of minutes to a few hours.

Question 30

What is the main nuclide used in diagnostic studies? How is it produced and why is it suitable?

Answer 30

^99mTc(V) is widely used and forms relatively intert complexes. It has a 6 hour half life and has a γ emission easily detectable in hospitals. [Tc] is proportional to blood flow and can build up a 3D image of clotting events. Radionuclides are obtained from nuclear reactors or cyclotrons. The desired isotope is then collected from an ion exchange column in the hospital. It is produced as [TcO₄]^- and is reduced to +V with Sn²⁺. Tc=O and O=Tc=O both form stable complexes with chelating ligands.

Question 31

Give three examples of chelating ligands that are used in conjuction with ^99mTc and the function the complexes serve.

Answer 31

Question 32

How are ¹¹¹In³⁺ and ⁶⁸Ga³⁺ used in diagnosis imaging?

Answer 32

¹¹¹In is used in SPECT (single photon emission computer topography), ⁶⁸Ga is used in PET. Both ions are hard cations so are bound by N or O donors.

Question 33

Describe the 3 main theraputic radiopharmaceuticals.

Answer 33

1. ⁹⁰Y³⁺, high energy β^- emitter suitable for cancer therapy. Hard cation chelated by DOTA. Co-administered with ¹¹¹In as cannot be directly detected. 2. ¹⁸⁶Re, very high energy β^- and γ emitter. Also matched for detection with ^99mTc. 3. α emitters are rarely used and dangerous to handle but can be extremely potent.