Reengineering Biopharmaceuticals for Targeted Delivery Across the Blood–Brain Barrier

Abstract

Recombinant protein therapeutics cannot enter brain drug development because these large molecule drugs do not cross the blood–brain barrier (BBB). However, recombinant proteins can be reengineered as BBB-penetrating IgG fusion proteins, where the IgG part is a genetically engineered monoclonal antibody (MAb) against an endogenous BBB receptor, such as the human insulin receptor (HIR) or the transferrin receptor (TfR). The IgG binds the endogenous insulin receptor or TfR to trigger transport across the BBB and acts as a molecular Trojan horse (MTH) to ferry into brain the fused protein therapeutic. The most potent MTH to date is a MAb against the HIR, designated the HIRMAb, which is active in humans and Old World primates, such as the Rhesus monkey. There is no known MAb against the mouse insulin receptor. For drug delivery in the mouse, protein therapeutics are fused to a chimeric MAb against the mouse TfR, designated the cTfRMAb. The HIRMAb or cTfRMAb Trojan horses have been engineered and expressed as fusion proteins with multiple classes of protein therapeutics, including lysosomal enzymes, neurotrophins, decoy receptors, single chain Fv therapeutic antibodies, and avidin. The pharmacokinetic (PK) properties of the IgG fusion proteins differ from that of typical MAb drugs and resemble the PK profiles of small molecules due to rapid uptake by peripheral tissues, as well as brain. The brain uptake of the IgG fusion proteins, 2–3% of injected dose/brain, is comparable to the brain uptake of small molecules. The IgG fusion proteins have been administered chronically in mouse models, and the immune response is low titer and has no effect on the fusion protein clearance from blood or brain uptake in vivo. The BBB MTH technology enables the reengineering of a wide spectrum of recombinant protein therapeutics for targeted drug delivery to the brain.

Introduction

Treatment of the brain with the products of biotechnology, for example, recombinant proteins and monoclonal antibodies (MAbs), or even short interfering RNA (siRNA), is not possible without fundamental solutions to the problem of blood–brain barrier (BBB) delivery. All biotechnological pharmaceutics are large molecules that do not cross the BBB. In the absence of an effective BBB drug delivery technology, the brain drug developer is left with the traditional, yet ineffective, brain drug delivery strategies, including transcranial drug delivery to the brain, BBB disruption, or small molecules. Transcranial drug delivery, such as convection-enhanced diffusion, only delivers drug to the local injection site (Pardridge, 2010) and is ineffective as a brain drug delivery technology for the 1200 g human brain. BBB disruption leads to chronic neuropathologic changes (Salahuddin et al., 1988) and is too toxic to be widely used in humans. Small molecules are hardly an alternative strategy because 98% of small molecules tested do not cross the BBB. To be brain penetrating, the small molecule must be lipid soluble, form < 8–10 hydrogen bonds with water, and have a molecular weight < 400–500 Daltons (Da) (Pardridge, 2005). Few small molecule pharmaceutical candidates have these molecular properties. Thus, even if a peptidomimetic small molecule were produced in lieu of drug development of a recombinant protein, the small molecule would most likely still need a BBB drug targeting technology to advance in clinical drug development.

The failure of biotechnology to treat the brain is illustrated with the biologic tumor necrosis factor (TNF)-α inhibitors (TNFI). Etanercept, the TNF receptor (TNFR); decoy receptor:Fc fusion protein, infliximab; the chimeric anti-TNFα MAb; and adalimumab, the human anti-TNFα MAb, had combined revenues of $16 billion in 2008 (Tansey and Szymbowski, 2009). However, there was no penetration of the CNS markets with the biologic TNFIs, despite the primary role played by TNFα in chronic brain diseases such as Alzheimer's disease (AD) or Parkinson's disease (PD) (Park and Bowers, 2010). The biologic TNFIs have not been developed for the brain because these molecules do not cross the BBB, and no BBB drug targeting technology has been developed within the pharmaceutical industry. The purpose of this chapter is to review the BBB molecular Trojan horse (MTH) technology for BBB-targeted delivery of recombinant protein pharmaceuticals.