Optimizing Antibody Internalization Assays: pHAb Dyes

22255190_pHabAmine_3D_image_050615-final-largePromega has recently developed a method that allows antibodies to be screened for their internalization properties in a simple, plate-based format. The method uses pH sensor dyes (pHAb dyes), which are not fluorescent at neutral pH but become highly fluorescent at acidic pH. When an antibody conjugated with pHAb dye binds to its antigen on the cancer cell membrane, the antibody-dye-antigen complex is not fluorescent, but upon internalization and trafficking into endosomal and lysosomal vesicles the pH drops, and the dye becomes fluorescent.

To demonstrate the broad utility of the pHAb dye for receptor mediated antibody internalization, two therapeutic antibodies, trastuzumab and cetuximab,which bind to HER2 and EGFR respectively, were selected for a case study (1). Both the antibodies, which are known to internalize were labeled with pHAb dyes using amine or thiol chemistry.

Parameters such as the impact of dye–to-antibody ratio on the antigen–antibody binding, change in fluorescence as a function of pH of free dye and labeled dye, and labeled antibody internalization as a function of pHAb conjugated antibody concentration were evaluated.

The results indicate that pHAb dyes are pH sensitive fluorescent dyes that enable the study of receptor-mediated antibody internalization.Internalization assays can be performed in a plate-based homogeneous format and allow endpoint assays as well as real-time monitoring of internalization. They further show that internalization can be monitored even at a very low amount of antibody which is very important during the early monoclonal antibody development phase when the amount of sample is limited and the antibody concentration in the samples is low. a complimentary approach, they  also showed that a secondary antibody labeled with pHAb dye can be used instead of labeling primary antibodies.

Literature cited

Nath, N. et al. (2016) Homogeneous plate based antibody internalization assay using pH sensor fluorescent dye J.  Immunol. Methods epub ahead of print

pH Reactive Dyes for Screening Antibody Internalization

19244302_phabaminebanner_900x246Antibody drug conjugates (ADCs) are a new class of therapeutic drugs that uses antibodies to deliver highly toxic drug molecules specifically to the cancer cells. A key requirement for ADCs is the ability of antibody to bind to the cancer cells followed by internalization and subsequent release of drug inside the cells leading to cell apoptosis.

Traditionally, selection of lead antibody candidates for ADCs was done in a sequential workflow where antibodies were first selected based on their affinity followed by characterization involving antibody internalization and drug conjugation. However, there is evidence that high affinity doesn’t always correlate with good internalization and hence there is a need to screen antibodies for internalization properties in addition to their affinities.

Promega has developed a method that allows antibody to be screened for their internalization properties in a simple, plate-based format. The method uses pH sensor dyes (pHAb dyes), which are not fluorescent at neutral pH but become highly fluorescent at acidic pH. When antibody conjugated with pHAb dye binds to its antigen on the cancer cell membrane they are not fluorescent but upon internalization and trafficking into endosomal and lysosomal vesicles the pH drops and dye becomes fluorescent.

Fluorescence signal, for pHAb dyes conjugated using either amine or thiol chemistry, is minimal at pH>7 and increase significantly as the pH drops to pH 5.0, which is a typical pH in cell endosomal compartment. Moreover, pH response of free pHAb dye is similar to that of conjugated dye indicating that conjugation chemistry doesn’t influence the pH response of the dye.

Due to the high signal-to-background ratios of the dyes, plate-based internalization assays can be performed, enabling screening of large libraries of antibodies for their internalization properties, hopefully leading to improved identification of lead candidates for ADC applications.