Figure 4. Gravitational sweep AUC analysis for a VLP conjugate formulated with low and high salt concentrations. The centrifugation
speed was varied from 3,000 to 20,000 rpm. The size distribution profiles were obtained using SedAnal.
Another example of applying gravitational sweep AUC is in formulation development for a Qβ virus-like-particle (VLP) conjugated
with a peptide at multiple sites. In a formulation matrix with higher salt concentration, SEC analysis suffered significant
loss in total peak area, and AF4 suggested the presence of very large aggregates (data not shown). To confirm, the VLP conjugate
in low and high salt formulations were analyzed with gravitational sweep AUC (Figure 4). The distribution profiles are markedly
different. The protein formed very large and heterogeneous aggregates in the high salt formulation that peaked at ~665 S.
The minimum radius and the minimum molecular weight for the equivalent anhydrous sphere are ~32 nm and ~111 MDa, respectively.
Particles of this size may not be recoverable by the matrix used in SEC chromatography.
Figure 5. Dynamic light scattering for drug substance batches produced at different manufacturing sites or by different manufacturing
processes. The scattered light intensity was measured at the scattering angle of 173°. The size distributions were analyzed
using the CONTIN algorithm, and averaged over multiple measurements.
DLS is another powerful biophysical technique that offers different advantages in characterizing aggregates. DLS measurement
is simpler and offers higher throughput than AUC, and requires less substantial sample dilution, and is more sensitive to
trace amounts of very large particles than SEC and AUC, which makes DLS very useful in detecting large aggregates. Figure
5 presents a DLS analysis of the bacterial Qβ VLP samples to evaluate a downstream process change. The process change did
not generate any large aggregates. However, despite the fact that the size distribution is monomodal, the peak has shifted
significantly to a larger size, suggesting the presence of small aggregates that are not resolvable by DLS. AUC–SV analysis
showed similar results (Figure 6). On the other hand, no substantial difference was detected by either DLS or AUC in samples
produced at different manufacturing sites using the original process, indicating that the original process is robust with
regard to site change.
Figure 6. AUC–SV for drug substance batches produced at different manufacturing sites or by different manufacturing processes.
The samples were analyzed at a centrifugation speed of 20,000 rpm and data were analyzed using Sedfit. Each profile is the
average of two replicates.