The polydispersity index (PDI) is one of the important parameters that characterize the uniformity of particle distribution, and it is the actual particle size distributionDeviation measurement between ideal monodisperse system (i.e. all particles have the same size)The value of PDI is usually measured by dynamic light scattering (DLS), reflecting the width and uniformity of particle size distribution.

The calculation formula for PDI is as follows:

Among them, σ is the standard deviation of particles, and ZD represents the light intensity weighted average particle size. The numerical range of PDI is generally between 0 and 1:

PDI=0: indicatesMonodisperse, which refers to the size of all particlesSame.

PDI<0.1: indicates a highly homogeneous dispersion system.

PDI>0.5: indicates a wide particle size distribution and a high degree of polydispersity in the system.

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Particle size distribution width (SPAN) is another indicator that characterizes the width of particle distribution.SPAN is usuallyUsed to characterize particle systems with asymmetric distributions, especially when the particle diameter distribution exhibits significant skewness.

The calculation formula for SPAN is as follows:

Among them, d10, d50, and d90 represent the particle size values at which the cumulative distribution of particle size reaches 10%, 50%, and 90%, respectively. The larger the SPAN value, the wider the particle size distribution.

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In the evaluation of nanoparticle size distribution, conventional PBE analysis is based on D10, D50, D90, and SPAND50 and SPAN are the main BE parameters, while D10 and D90 are the supporting parameters; At present, whether as an option or a requirement, Z-average and PDI are only used as alternatives to D50 and Span, as alternative solutions for certain products (such as DLS devices) when SPAN cannot be provided; Compared with SPAN, PDI amplifies the difference in T/R (test drug and reference drug); Compared to SPAN, using PDI may be more difficult to evaluate through PBE.

4. Different numerical ranges and standards:

The numerical range of PDI is between 0 and 1, mainly used to evaluate the uniformity of particle distribution. SPAN, on the other hand, does not have a fixed range and focuses more on the width of particle distribution.

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For scientific research and industrial applications, selecting appropriate indicators for characterization helps to better understand and control the characteristics and behavior of nanoparticles. Overall, PDI and SPAN are important indicators for evaluating the particle size distribution of nanoparticles, and they have their own advantages and applicability in different application scenarios.

From the perspective of scientifically evaluating PBE in current drug development, PDI is not an appropriate parameter, nor is it the correct choice to replace SPAN.PDI may only be applied to drug particle evaluation with the widespread use of DLS technology, but it is used for drug delivery of extracellular vesiclesLipid NanoparticlesSamples of biological particles such as viral particles are often highly dispersed, while DLS and others are based on particlesBrownian motionStrictly speaking, the representation method is not applicable. Therefore, the National Medical Products Administration (NMPA) Drug Evaluation Center also suggested in the document to accumulate research data on particle cumulative distribution D10, D50, D90, SPAN, and total particle count, in addition to the average particle size. Due to the fact that the DLS method calculates the fitted average particle size (Z-average), it is not possible to provide the parameters of particle cumulative distribution D10, D50, D90. Therefore, it is recommended to use multiple detection methods for in-depth research on nanoparticles^[1].

Through the introduction of this article, we hope that everyone has a clearer understanding of PDI and SPAN, and can more accurately select and use these indicators in future research and applications.