Antibody Development: A Question of Balance

Therapeutic monoclonal antibody (mAb) development is a balancing act that pits a drug’s performance (safety, effectiveness, pharmacodynamics (PD), pharmacokinetics (PK), and return on investment) against practical concerns related to promoting hits to candidates, from the test bench to doctors’ offices and clinics.

Heading the list for practical or engineering considerations are developability and manufacturability. These two factors are related through stability and solubility—two properties whose optimizations are sometimes at odds.

Developability

A 2013 study published by scientists from Merck described developability as “an important link between drug discovery and process development", adding that “early assessment of the molecule’s stability is the first occasion to gather knowledge on it to determine potential CQAs [critical quality attributes] as encouraged by the QbD [quality by design] paradigm.”

In making the case for improving manufacturability by conducting developability studies before process development, the authors discussed stability only from perspectives of in-process durability and shelf-life, but a product’s circulating half-life is a type of stability as well. Developers need to know, whether their product sits in a refrigerated vial or is circulating in the bloodstream, that it will break down or be otherwise disposed of in regular, predictable ways.

Developability, according to a later study (also from Merck), refers to the feasibility or ease of promoting mAbs from discovery to development by evaluating their physicochemical properties, particularly self-interaction and aggregation, thermal stability, colloidal stability, and the potential to optimize a structure through sequence engineering.

These authors wrote that early-stage assays evaluating a mAb’s drug-like properties can help development teams select lead candidates “with optimal physicochemical and biophysical properties” which “would save development costs and accelerate the timeline toward first-in-human clinical initiation, eventually leading to the development of best-in-class drugs in terms of effectiveness, administration route, circulating half-life, and dosage.”

Manufacturability

Biologics manufacturability is a huge topic that affects mAb development at every stage and persists into post-marketing activities. Manufacturability is a simple concept—how easily can a quality product be produced at scale? —but its inputs and outputs are varied and often interdependent.

For example, yield and quality depend on cell culture conditions (feeds, media, batch or continuous, single-use or fixed-tank) but also on factors difficult to control like the expression system. Most bioprocesses take the biggest hits in yield during downstream operations, but even when the right process is available it may not be feasibly deployed due to cost, time, scheduling of unit operations, or lack of the appropriate expertise.

In some cases, solubility, stability, pharmacokinetic, and even pharmacodynamic  issues identified during a developability assessment are not fully resolved and persist into pilot-scale production.

One way to minimize their impacts is by taking a translational approach to PK/PD, which uses mechanism-based models to predict a drug’s disposition in humans through the translation of chemical and animal test results to humans.

Translational PK/PD involves obtaining appropriate efficacy, safety, PK, and PD data from preclinical studies, establishing dose-response PK/PD relationships, predicting human PK, and finally integrating PK, safety, and efficacy data to determine dose ranges.

Several translational PK/PD methods have sprung up, and nearly every preclinical-stage mAb developer adds their own wrinkles. Some recent translational PK/PD mAb developments include target-mediated drug disposition, physiologically based pharmacokinetic models, and quantitative systems pharmacology.

In vitro-in vivo correlation, another type of translational PK/PD, was recently used to develop M8891, a reversible inhibitor of methionine aminopeptidase 2, an enzyme implicated in cancer, obesity, and autoimmune diseases. Under the sponsorship of Merck KGaA, M8891 is currently in a phase 1 dose-escalation study in patients with advanced solid tumors.

One company’s perspective

To John Cardone, Marketing Manager for Custom Antibody Services at Bio-Rad, the critical considerations in antibody optimization are specificity, affinity, and improving developability. But since these factors are interrelated, improving one usually affects the others, and not always to advantage.

Screening and testing a large number of clones leads to the identification of candidates that excel according to screening or testing specifications. But antibody candidates displaying the best functionality often have suboptimal biophysical properties, which negatively affect scaleup and manufacturability.

“Improving these suboptimal parameters may, in turn, negatively impact the original affinity and functionality of the lead candidates,” Cardone says.

Expert design of an antibody generation platform can help overcome these interdependent factors. According to Bio-Rad, its Pioneer™ Antibody Discovery Platform delivers candidate antibodies with high affinity and diversity, and with excellent developability, within a short timeline.

“There are several layers to this,” Cardone explains. The Pioneer platform is optimized for phage display, and codon usage (i.e., the antibody expression system) has been designed for high, balanced expression of all clones occurring in the antibody library.

Pioneer was engineered to deliver antibodies with good developability by reducing the amino acid motifs negatively affecting manufacturability (in part through post-translational modifications, by eliminating framework mutations, and by using defined germline genes known with favorable properties.

“We achieved this through an exceptionally large library that delivers large numbers of specific antibodies with good epitope coverage and developability,” Cardone says. “Combined with improved phage display selection, large library size directly leads to antibodies with affinities in the sub-nanomolar range, right out of the library.

Pioneer Antibody Discovery works with SpyDisplay, a Bio-Rad phage display technology that uses a single vector for both phase display and clone expression, thereby improving efficiency.

“Traditional methods need to subclone the selected antibody genes into a suitable expression plasmid for further analysis,” Cardone tells Biocompare. “SpyDisplay avoids subcloning steps.”

The process is not automatic and does not produce candidates out-of-the box. Antibody optimization must still be tailored to the sponsor’s specific requirements by applying screening and functional testing steps, including developability assessment.

“But once these constraints are applied, Pioneer routinely generates antibodies optimized to roughly the level of a late clinical-stage molecule,” Cardone says, “which helps overcome challenges in discovery-stage antibody optimization related to specificity, affinity optimization, and developability.”