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Guest Editor - Steve Shire
Is Any Measurement Method Optimal for All Aggregate Sizes and Types?
Submitted: January 24 , 2006 ; Accepted: June 22 , 2006 ; Published: September 8 , 2006
John S. Philo 1
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2. 2
Modern methods of sedimentation velocity data
analysis1 allow us to separate and quantitate long-lived
aggregates with high resolution and sensitivity, giving a
distribution much like a chromatogram.
1Schuck, P. (2000). Size-distribution analysis of macromolecules by
sedimentation velocity ultracentrifugation and Lamm equation modeling.
Biophys. J. 78, 1606-1619.
5.9 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7.0 7.1 7.2
Radius (cm)
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
Absorbanceat280nm
0 2 4 6 8 10 12 14 16 18 20 22 24
0.0
0.2
0.4
0.6
0.8
1.0
1.2
c(s),normalized
sedimentation coefficient (Svedbergs)
raw data sedimentation coefficient distribution
3. 3
THE PROMISE
These data for a highly stressed antibody sample illustrate excellent
resolution, sensitivity, and range of size
0 2 4 6 8 10 12 14 16 18 20 22 24
0.0
0.2
0.4
0.6
0.8
1.0
heptamer,0.1%
hexamer,0.4%
pentamer1.4%
tetramer5.3%
trimer14.6%
dimer30.6%
main peak (monomer), 45.5%
?HLhalfmolecule,0.8%
?freelightchain,1.4%
c(s),normalized(totalarea=1)
sedimentation coefficient (Svedbergs)
4. 4
THE PERIL
The similarity of these distributions to chromatograms
can be deceptive, and the software is easily miss-applied
1. the effective resolution is a function of signal/noise
ratio and goes down as the fraction of minor peaks
goes down
– the resolution you can achieve for a 150 kDa antibody is
also much greater than for a 20 kDa cytokine
2. the nature of the noise (variability) is very different
than in chromatography
3. false peaks are possible, especially when the
software is applied inappropriately
8. 8
Protein Z: trace peaks (<0.05%) in multiple lots fall at
similar positions, implying these are real species
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
0
2
4
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
0.00
0.01
0.02
normalizedc(s)
sedimentation coefficient (Svedbergs)
lot 1
lot 2
0.03%
0.03%
0.04%
0.04%
0.70%
0.16%
X 200
9. 9
Changes in a lyophilized protein over time after re-hydration imply
a peak initially detected at a level of only 0.007% is a real species
1 10 100
0.00
0.06
0.12
0 hr
2 hr
8 hr
s×c(s)
sedimentation coefficient (Svedbergs)
0.02%
0.07%
0.38%
0.007%
X200
10. 10
How can we predict the expected detection limits
and reproducibility? We can exploit the fact that
sedimentation velocity has a robust theory!
1. use the theory to simulate data for a known mixture, for
example monomer + known percentages of irreversible
aggregates
mimic the data acquisition schedule, loading concentration, and
noise level of real experiments
2. analyze the simulated data applying the same procedure
as in real experiments, and see how well the results
match the mixture that was simulated
3. repeat 1 & 2 several times to evaluate reproducibility
12. 12
What sources of systematic error can prevent
achieving high reproducibility and sensitivity?
1. Detergent micelles may produce peaks that overlap with
species of interest
Tween-80 micelles sediment around 2 S, and can often be detected
by absorbance, particularly at wavelengths below 280 nm
poor cancellation of detergent signals when detergent is present in
both sample and reference channels is common
2. High levels of excipients (sucrose, sorbitol, amino acids)
may produce significant density gradients at high r.p.m.,
slowing the sedimentation as the run proceeds
in principal new models in the software can handle this, but we
often lack the required data on the density and viscosity effects of
the excipients
13. 13
Other problems and sources of error, continued
3. Some proteins are easily damaged by UV exposure
SV protocols now typically involve many more scans than they did
in the past and hence much more UV exposure
remember, the entire sample is exposed to light with every flash, not
just the radius being measured at that time
4. Aggregates are generally quite sticky, and may get lost
during transfer (on surfaces of vials, pipette tips, or the
centrifuge cell)
in theory multiple rinses should minimize this problem, if sufficient
sample is available
14. 14
Conclusions
1. Both experiment and theory indicate that individual
aggregate species can be measured with a reproducibility
of ±0.1% or better in some cases
• resolution and reproducibility is generally higher for larger
proteins like antibodies than for small cytokines
• precision is poorer when many poorly-resolved aggregate species
are present
• it is normal for the positions of minor peaks to shift from sample
to sample, and the software sometimes merges or omits minor
peaks that are near the detection threshold
2. Large aggregates that sediment > ~3 times faster than
monomer can generally be detected at levels of 0.1% or
below
3. The levels of precision and sensitivity illustrated here
cannot necessarily be achieved for all proteins, or by
relatively inexperienced analysts