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PH3505- Medicinal chemistry > Lec 2- HPLC > Flashcards

Lec 2- HPLC Flashcards

1
Q

Analysis of drugs by HPLC

A
  • Solvents => Pump => Injector => Column => Detector => Recorder
  • Can be considered as a modular system
  • How many stationary phases (Based on the mechanism of separation) are there in LC?
  • Which is most commonly used?
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2
Q

Using reverse phase HPLC

Parameter changed= Column length

A
  1. Increased column length
  • Results in Increased direction of change
  • And Increased retention time
  • Increased Resolution ​
  1. Decreased Column length
  • Decreased direction of change
  • Decreased retention time
  • Decreased resolution
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3
Q

Using reverse phase HPLC

Parameter changed= Flow rate

A
  1. Increased flow rate
  • Increased direction of change
  • Similar retention time- often decreased retention time
  • Decreased resolution
  1. Decreased flow rate
    * Decreased direction of change
    * Similar retention time- Often increased retention time
    * Increased resolution

NB- The degree of separation is most easily controlled by changing a mobile phase composition

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4
Q

Determination of methotrexate in plasma by HPLC

A
  • What diseases do the first 2 drugs treat
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5
Q
  1. Clean up procedures

Q1) Where was the other 60% of the MTX?

Q2) Was this likely to cause errors?

Q3) How can these errors be compensated?

A
  • Protein was precipitated by adding 0.3 ml of 10% trichloroacetic acid to every 1 ml of plasma
  • Following centrifugation, MTX could be measured with a UV detector at 305nm
  • Recovery of MTX in the acid extracts was reproducible at about 40% (38 to 41% in 9 determinations)

Q1) Where was the other 60% of the MTX?

  • It is likely that it was bound to the precipitated protein

Q2) Was this likely to cause errors? - Yes

Q3) How can these errors be compensated? By adding internal standard

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6
Q
  1. Derivatisation

Q1) Why were the drug and IS oxidised

A
  • For each 0.5ml of trichloracetic acid extract, 0.05ml of 5M acetic acid- Na acetate buffer (pH 5.0) was added
  • Oxidation was performed at room temperature in 5 minutes by the addition of 5% KMnO4 (aq) solution
  • The samples were decolourized by the addition of 0.1 ml of a 3% solution of H2O2
  • This procedure resulted in the quantitative conversion of MTX to 2,4-diaminopteridine-6-carboxylic acid
  • When 2-hydroxyfolic acid was used as the internal standard it was added to the trichloroacetic acid solution to give a final concentration equivalent to 0.5ug/mL in plasma
    • To convert them into fluorescent derivatives to improve the detection sensitivity
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7
Q
  1. Analysis
  • Q1) What was the stationary phase?
  • Q2) Was it reverse phase or normal phase HPLC
  • Q3) Tris?
A
  • Water 6000 pumps were used with a uBondpack C18 column (4mm x 30cm) and a model U6K injector
  • A SF-770 multiple wavelength UV detector and model FS (fluorescence detector) 970 detector were connected in series to the outlet of the column
  • 2 solvent systems were used
  • Q1) C-18
  • Q2) A reverse phase
  • Q3) Tris(hydroxymethyl) aminomethane: (CH2OH)3CNH2
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8
Q
A

NB look at BB

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9
Q
  1. HPLC trace
A
  • The HPLC traces for MTX analysis by HPLC and fluorescence detection
  • Samples of human plasma were ‘spiked’ with MTX (0.1 ug/ml) and 2-hydroxyfolic acid (0.5 ug/ml)
  • Samples (100 ul) of oxidised plasma extracts were loaded onto the chromatograph A
  • Plasma plus MTX and the internal standard; B
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10
Q
  1. Calibration curve
A
  • Plasma containing 2-hydroxyfolic acid (0.5 ug/ml) and different levels of MTX was extracted, derivatized and analysed as discussed (6 samples, each injected 3 times )
  • MTX plasma standard curve with the use of an internal standard
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11
Q

Assay procedure of paracetamol tablets using a calibrate curve

A
  1. Weigh out 125mg of paracetamol standard and dissolved in 250ml of 0.05M aq. acetic acid (stock solution)
  2. Prepare a series of standard solution each containing 0.5, 1.0, 1.5 & 2.5mg/ 100ml of the drug from the stock solution
  3. Weigh out powder 20 tablets (total 12.1891 g)
  4. Weigh out 150.5mg of tablet powder, shake the powder in ca and then adjust the volume to 250ml with more acetic acid (0.05M)
  5. Filter about 50ml of the solution into a conical flask and then transfer 25 ml of filtrate to 100ml volumetric flask and adjust the volume to 100ml
  6. Take 10ml of the diluted extract and transfer to a further 100ml volumetric flask and makeup to volume with 0.05M acetic acid
  7. Analyse the standard solutions and the extract from step 6 using the following chromatographic conditions: C-18: mobile phase= 0.05 acetic acid/acetonitrile (90:15); detection: UV- 243nm; flow rate 1ml/min
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12
Q

Data obtained from the analysis of paracetamol solution

A
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13
Q

Paracetamol example

A
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14
Q

Main steps to consider

A
  1. Chose a chromatographic method (HPLC or GC)
  2. Chose a Column
  3. What kind of detector
  4. Solvent system
  5. Wavelength
  6. Chose internal standard
  7. Evaluate the sensitivity of the assay
  8. Pre-treatment of the samples
  9. Analysis
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15
Q
  1. which chromatographic method would you use

Advantages and disadvantages

A
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16
Q
  1. what kind of column
A
  • Reverse or normal phase
  • C-18 or C8
17
Q
  1. what kind of detector
A
  • Which one to use depends on the structure of the drugs

Using UV detection

  • The drug must have reasonable chromophore for UV detection
18
Q
  1. what solvent system
A
  • Reverse phase= aq base organic solvents
  • Normal phase= organic solvents
  • If the drug being analysed can be ionised we must buffer the solution
19
Q
  1. what wavelength to use
20
Q
  1. which IS
A
  • Commercially available
  • Similar chemical and physical properties
  • Likely to be presenst in the clinical samples? if yes, dont use it
21
Q
  1. Evaluate sensitivity of the assay
A
  • HPLC after parameters have been optimised
  • Minimum amount of drug detectable: 10ng
  • Signal/noise: >3/1
22
Q
  1. Pre-treatment of the samples (to clean and concentrate)
A
  • Use of plasma: how are you going to clean up the sample?
  1. Chromatography
    • Ion exchange may be difficult- not strongly ionic
    • Preliminary RP chromatography e.g. SEP-PAK (solid phase extraction
23
Q

solid phase extraction

A
  • An efficient method for isolating and concentrating solutes from large volumes of liquid
  • Very effective, even when the solutes are present in very dilute concentrations (ppb)
  • Materials obtained can be used for chromatographic separation, spectroscopic examination or biological assessment
24
Q

The typical procedure for solid phase extraction

A
  1. Conditioning solvent- (sorbent)
  2. Analyte + sample matrix
  3. Sorbent + analyte + impurities
  4. Washing solvent- sorbent and analyte in extractor and impurities taken out
  5. Eluting solvent- analyte removed
25
Q

A typical procedure for solid phase extraction

A
  1. Conditioning the column: wash the column with a 5-10 bed volume of the solvent (same as to be used to elute the analyte
  2. Loading the analyte (liquid sample)
  3. The sample solvent passes through the column, the analyte + impurities absorbed on the solid phase
  4. Wash the column with a solvent, which will elute impurities but leave the analyte on the column
  5. Elute the analyte with an appropriate solvent to elute out the analyte (drug)
26
Q

B. extraction

A
  • Extraction with organic solvents
  • Try light ones first (organic phase stays on top of aqeous layer e.g. hexane, diethyl ether, ethyl acetate
  • Effect of pH on extraction
  1. Take 1 ml of buffer with drug and IS
  2. Add 3ml of diethyl ether
  3. Vortex for 30 sec
27
Q

If the recovery is satisfactory, then go through the same procedure using control plasma instead of buffer

28
Q

How can we get over the problem- back extraction

29
Q

Interfering compounds

A
  • Are there other drugs which the patient may be taking which could interfere with your assay
  1. Talk to the clinician about this
  2. If there are:
    • Run through these drugs with control plasma or
    • Get plasma from patients who are not taking these drugs
  • Also, check for interference with the internal standard
30
Q

Development of chromatographic assay (main steps)

A
  1. Chose a chromatographic method
  2. Chose a column
  3. What kind of detector
  4. Solvent system
  5. Wavelength
  6. Chose IS
  7. Evaluate sensitivity of the assay
  8. Pre-treatment of the samples
  9. Analysis
31
Q

Validation of analytical procedures- methodology

A
  • The extent of validation will depend on where a drug is in the development process
  • In USA(FDA)
    • Investigational new drug application- application for carrying out clinical trials
    • New drug application- application for sale and marketing of a new drug
  • In UK (MHRA)
    • Clinical trial authorisation application (for clinical trials)
    • Marketing authorisation application (for sale and marketing)

PH3505- Medicinal chemistry (29 decks)

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