There has been a significant resurgence in the development of antibody-drug conjugates (ADC) as target-directed therapeutic
agents for cancer treatment. Among the factors critical to effective ADC design is the Drug Antibody Ratio (DAR). The DAR
describes the degree of drug addition that directly impacts both potency and potential toxicity of the therapeutic, and can
have significant effects on properties such as stability and aggregation. Determination of DAR is, therefore, of critical
importance in the development of novel ADC therapeutics.
DAR is typically assessed by mass spectrometry (MALDI–TOF or ESI–MS) or UV spectroscopy. Calculations based on UV absorption
are often complicated by similarities in extinction coefficients of the antibody and small molecule. Mass spectrometry, though
a powerful tool for Mw determination, depends on uniform ionization and recovery between compounds — which is not always the case for ADCs.
Here we present a method for DAR determination based on SEC–MALS in conjunction with UV absorption and differential refractive
index detection. Figure 1 shows UV traces for two model ADCs; molecular weights of the entire ADC complexes are determined
directly from light scattering data.
Component analysis is automated within the ASTRA 6 software package by using the differential refractive index increments
(dn/dc) and extinction coefficients, which are empirically determined for each species or mined from the literature, to calculate
the molar mass of the entire complex as well as for each component of the complex.
Figure 1: Molar masses for two distinct ADC formulations are determined using SEC–MALS analysis.
In this example an antibody has been alkylated with a compound having a nominal molecular weight of 1250 Da (Figure 2). Molar
masses of the antibody fractions are similar, which indicates that the overall differences between the two formulations reflect
distinct average DARs that are consistent with values obtained by orthogonal techniques. Note that the molar mass traces for
the conjugated moiety represent the total amount of attached pendant groups; the horizontal trends indicate that modification
is uniform throughout the population eluting in that peak.
Figure 2: Molar masses for the antibody and total appended drug are calculated in the ASTRA software package based on prior
knowledge of each component’s extinction coefficent and dn/dc, allowing determination of DAR based on a nominal Mw of 1250
Da for an individual drug.
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