Discussion
The assessment of the continuous ILDF set up for liposomal drug product
formulation refinement was successful and provided a range of intriguing
results. The notion of the permeate being impacted by the ethanol
concentration in the batch process was reinforced by the results from
the continuous ILDF experiment(s).
The use of the simulated ILDF (Figure 1D) showed that reduction of the
initial ethanol concentration produced an ethanol concentration
dependent rate of reduction in the number of passes/stages necessary to
achieve the target ethanol removal. This led to the derivation of an
equation (Equation 2) for the simulated ILDF concentration/dilution
arrangement (Figure 1D) including the derivation of a function for
ethanol concentration dependency of the rejection coefficient (Equation
3).
While these equations prove applicable and predictable under the
simulated ILDF set up from Figure 1D, these equations proved less
applicable to a real-world set up such as that shown in Figure 1C. The
simulated ILDF method showed that the permeability of the hollow fiber
was limited by the initial exposure of ethanol. The permeability then
only improved a limited amount as the ethanol concentration decreased
with each pass. By using the same hollow fiber to simulate each pass of
a ILDF system, the efficiency of each subsequent pass was limited and
not representative of a true ILDF arrangement. By using the data from
the initial passes of the separate runs, a true ILDF data set was
extrapolated and assessed.
The continuous ILDF model showed an almost linear reduction as compared
to the exponential reduction in the simulated ILDF model (Figure 4B vs.
Figure 3B). This may be because the permeability of the hollow fibers in
the continuous ILDF model were set independently and had no impact on
the subsequent hollow fibers as the ethanol concentration decreased.
Additionally, the linear reduction of the ethanol in the continuous ILDF
model limited the overall buffer consumption to that of a batch process
where the rejection coefficient is zero. This showed that continuous
ILDF would be more efficient than a solvent-based batch TFF with respect
to buffer required.
Based on these findings, the impact of ethanol on the continuous ILDF
design is less driven by a traditional rejection coefficient concept and
more on ethanol’s effect on hollow fiber permeability. By reducing the
initial exposure of each individual hollow fiber in a continuous ILDF
system, the performance and overall efficiency of the system is
improved. The most optimal design for a liposomal continuous ILDF
process would involve significant upfront dilution (i.e. 5% initial
ethanol concentration) in order to start the ILDF with minimal ethanol
concentration. This would minimize the amount of ILDF stages needed and
buffer required. This is similar to the dilution strategy recommended
for albumin diafiltration though optimization is not specifically
correlated to the impact of ethanol concentration on permeability
(Jaffrin et al. 1994, Paulen et al. 2011). Additionally, these finding
may prove applicable and beneficial to mRNA-LNP vaccines, which often
look to minimize mRNA exposure to organic solvents as well as having
massive production demands (Hou et al. 2021, Schoenmaker et al. 2021,
Verma et al. 2023), which a continuous process design could help to
meet.