SCIENCE & TECHNOLOGYCOVANCEDRUG DILIGENCEAnalysts p erformbioassays in cell-culturehoods at Covance’s NorthAmerica BiotechnologyServices facility.ASSAYING ANTIBODIESDrug manufacturers pinpoint TECHNIQUES TO ANALYZEgrowing and diverse class of therapeuticsJYLLIAN KE MSLEY , C&EN WEST C OAST NE WS B UREAUMONOCLONAL-ANTIBODY-based drugsare a large and growing segment of pharmaceuticals.Since murom<strong>on</strong>ab-CD3(Orthocl<strong>on</strong>e OKT3) was first approved bythe Food & Drug Administrati<strong>on</strong> in 1986 asan antirejecti<strong>on</strong> drug for organ transplantpatients, more than 30 antibody-baseddrugs have entered the market, and manymore are in the pipeline. In 2011 al<strong>on</strong>e,FDA approved belimumab (Benlysta) forlupus, ipilimumab (Yervoy) for metastaticmelanoma, and brentuximab vedotin (Adcetris)for two types of lymphoma. Marketresearch company Datam<strong>on</strong>itor estimatesthat sales of antibody therapeutics willgrow by 8.2% from 2010 to 2016, the fastestof any therapeutic class, with sales expectedto surpass $65 billi<strong>on</strong> by 2016.Although drugs based <strong>on</strong> m<strong>on</strong>ocl<strong>on</strong>alantibodies—MAbs—have widely variedtherapeutic acti<strong>on</strong>, they are all based <strong>on</strong> thesame protein, immunoglobulin G (IgG).But this shared attribute does not meanthat these drugs are simple to assay. MAbsare composed of more than 1,000 aminoacids in four peptide chains, and they bearsugars and other chemical modificati<strong>on</strong>s.As the MAb class grows, scientists aresettling <strong>on</strong> some standard critical proteinqualities and analytical testing approachesfor drug identity, quality, and purity forthese large, complex molecules.Al<strong>on</strong>g those lines, the U.S. PharmacopeialC<strong>on</strong>venti<strong>on</strong> (USP), the pharmaceuticalstandard-setting organizati<strong>on</strong>in the U.S., is in the midst of developing“best practice” guidelines and regulatorystandards for MAb drug developers. “Theindustry has more than 20 years of manufacturingexperience with these types ofmolecules, and we’re seeing products thatare extremely pure and really well characterized,”says Tina S. Morris , USP’s vicepresident for biologics and biotechnology.“Every antibody has slightly differentpurificati<strong>on</strong> and analytical characteristics,but you’re never starting from zero with acompletely unknown protein.”One set of standards, to be known asChapter 1260, will <str<strong>on</strong>g>focus</str<strong>on</strong>g> <strong>on</strong> how to develop,define quality attributes for, and manufactureMAbs.These standards will not be legallybinding <strong>on</strong> MAb manufacturers but insteadare intended to provide worldwidebest practice guidance, says Anth<strong>on</strong>yMire-Sluis, corporate vice president forproduct and device quality at Amgen andchair of the USP Recombinant TherapeuticM<strong>on</strong>ocl<strong>on</strong>al Antibodies Expert Panel,which is developing the standards. Theexpert panel includes 13 industry members,as well as seven representatives fromregulatory and other standard-settingagencies.The other set of standards, Chapter 129,will be legally binding <strong>on</strong> MAb manufacturersand outlines the minimal quality attributescomm<strong>on</strong> to all antibodies and thecorresp<strong>on</strong>ding recommended analyticaltests for MAb therapeutics. Several companieswill validate the proposed methodsWWW.CEN-ONLINE.ORG 16 MARCH 2012
to ensure that they work, and the standardswill also include limits derived frommanufacturer data <strong>on</strong> actual products. But,Mire-Sluis notes, companies will be ableto justify using different tests or limits aspart of their drug applicati<strong>on</strong> package ifthey think their product merits somethingatypical.STRUCTURALLY, an antibody,or immunoglobulin, is a large,Y-shaped protein composedof two “heavy” and two “light”peptide chains. The heavychains come together at thebottom half of the Y, also calledthe crystallizable fragment, orFc. Disulfide b<strong>on</strong>ds and n<strong>on</strong>covalentinteracti<strong>on</strong>s hold thechains together. This part ofthe antibody has a c<strong>on</strong>servedamino acid sequence and bindsto receptors to activate partsof the immune system. Differentantibody classes—IgA,IgD, IgE, IgG, and IgM—havedifferent heavy chains that determinespecific immunologicalroles. IgG provides most of thebody’s immunity to invadingpathogens.At the top half of the antibodyY, the heavy chains branchout and each associates witha light chain, also through disulfideb<strong>on</strong>ds and n<strong>on</strong>covalentinteracti<strong>on</strong>s. This regi<strong>on</strong> of the antibodyis called the antigen-binding fragment,or Fab, and it is here that the amino acidsequence varies to bind to assorted foreignmolecules: Both arms of a specific antibodyhave the same amino acid sequence, whilethe variable porti<strong>on</strong> differs between differentantibody molecules.But because MAbs are made by cl<strong>on</strong>edcell lines with identical cells, all of the antibodiesproduced by a cell line engineeredfor producing a particular therapeuticshould be identical. M<strong>on</strong>ocl<strong>on</strong>al IgG therapeuticswere originally based <strong>on</strong> mouseantibodies, but they now can be chimeric,which means a part-mouse/part-humansequence, or “fully” human. Structurally,the three types are very similar, but theymay provoke different immune resp<strong>on</strong>sesin patients.In additi<strong>on</strong> to their basic structurallikeness, MAb therapeutics also tend tobe purified similarly, USP’s Morris notes.Purificati<strong>on</strong> typically starts with affinitychromatography using a solid phase thatanchors protein A, a bacterial proteinthat selectively binds to the antibodies’c<strong>on</strong>served Fc regi<strong>on</strong>. After that, the drug islikely treated to inactivate any c<strong>on</strong>taminatingviruses and may undergo fine-tuningpurificati<strong>on</strong> steps. Most antibodies arealso currently produced from two popularcell lines, which means that their impurityCONFIGURATION The basic IgG structure involvestwo heavy chains and two light chains held together bydisulfide b<strong>on</strong>ds and n<strong>on</strong>covalent interacti<strong>on</strong>s. The proteinmay be glycosylated at a c<strong>on</strong>served site (shown) orelsewhere <strong>on</strong> the structure.SSSSSSOligosaccharideHeavy chainVariable ◼C<strong>on</strong>served ◼Light chainVariable ◼C<strong>on</strong>served ◼SSSSSSSSprofiles are similar. The combinati<strong>on</strong> ofsimilar cell culture and purificati<strong>on</strong>, <strong>on</strong> topof targeting the same protein, further lendsanalysis of MAb therapeutics to similarprotocols, Morris says.But despite these similarities, MAbsmay show heterogeneity depending <strong>on</strong>how they were produced and processedby their originating cells. Problems withDNA translati<strong>on</strong> or transcripti<strong>on</strong> may leadto different protein sequences. Proteinsmay misfold or mismatch their disulfideb<strong>on</strong>ds. Posttranslati<strong>on</strong>al modificati<strong>on</strong> mayproduce different glycosylati<strong>on</strong> patterns.Side chains may also be subject to chemicalreacti<strong>on</strong>s, such as oxidati<strong>on</strong> or deamidati<strong>on</strong>.Proteolytic enzymes may clip theprotein, especially the lysines at the ends ofthe heavy chains in the Fc. MAbs may alsodenature or aggregate as they go throughpurificati<strong>on</strong> or formulati<strong>on</strong>.Because of these effects, even a perfectlyprepared MAb therapeutic will includeIgG proteins with varied structuresWWW.CEN-ONLINE.ORG 17 MARCH 2012SSSSSSSand properties, says William Whitford,senior market manager for cell culture andbioprocessing at Thermo Fisher Scientific .“Unlike small-molecule drugs, biologicsshow a distributi<strong>on</strong> of slightly differentmolecules,” he says. The analytical challengeis therefore to ensure that the populati<strong>on</strong>distributi<strong>on</strong> doesn’t vary significantlyfrom the drug that was developed,tested clinically, and approved.SOligosaccharideSSSOME PROPERTIES are aprocess red flag. Oxidati<strong>on</strong> ordeamidati<strong>on</strong>, for example, byitself generally doesn’t affectan antibody’s clinical functi<strong>on</strong>,says Raym<strong>on</strong>d Kaiser,global science leader and vicepresident for biotechnologyservices at Covance , a c<strong>on</strong>tractdrug development company.But variability in those categoriesis usually a sign thatthe manufacturing process isnot well c<strong>on</strong>trolled, and otherprotein characteristics, such asaggregati<strong>on</strong>, may be affected,Kaiser says.Other modificati<strong>on</strong>s are aclinical c<strong>on</strong>cern. A misfoldedprotein or <strong>on</strong>e with abnormaldisulfide b<strong>on</strong>ds probably w<strong>on</strong>’thave the structure needed tofuncti<strong>on</strong> properly. Additi<strong>on</strong>ally,IgGs have a c<strong>on</strong>servedglycosylati<strong>on</strong> site <strong>on</strong> the Fc,but variati<strong>on</strong>s in those sugars and glycosylati<strong>on</strong>elsewhere <strong>on</strong> the protein may elicitan unwanted immunological resp<strong>on</strong>se,such as anaphylaxis. Protein aggregatesmay also provoke an immune resp<strong>on</strong>se.And so a suite of analytical tests is necessaryto ensure that a MAb therapeuticis within the bounds of the drug that wasoriginally prepared, tested, and approved.“You live and die by your analyticals,” Kaisersays. “If you d<strong>on</strong>’t have your analyticals,you d<strong>on</strong>’t know anything.”The first order of business is often toidentify whether a manufacturer has madethe correct protein. One comm<strong>on</strong> approachis to do an immunoassay, such asan enzyme-linked immunosorbent assay(ELISA). In these types of assays, a stati<strong>on</strong>arysolid phase is loaded with a moleculethat will bind specifically to the target antibody.A drug sample is loaded, incubated,and washed, and some sort of label is usedto assay whether and how much of the targetantibody is bound.