Leading the way in bringing gene therapy to the CNS with conditioning: a renaissance for busulfan?

Chris Mason, MD, PhD, FRCS, FMedSci
Chris Mason, MD, PhD, FRCS, FMedSci
Chief Scientific Officer

Effective treatment of the central nervous system (CNS) through gene therapy is a promising approach for the many genetic diseases with neurological manifestations. It’s a particular hurdle for diseases with both CNS and systemic involvement, where an effective therapy would need to work on both sides of the blood–brain barrier. Prominent among this group are lysosomal disorders (LDs), a family of monogenetic degenerative diseases which we focus on at AVROBIO, some of which have severe neurodegenerative aspects[1].

Effectively arresting or perhaps even reversing this degeneration could spare parents the pain of watching their child fail to progress or even regress through milestones – as seen in young boys with Hunter syndrome[2], for example. It could prevent many adults from developing genetically linked dementias such as GBA-Parkinson’s disease – seen disproportionately in people with Gaucher disease[3]. The medical need is inarguable and is driving intense research. My colleague Geoff MacKay recently reviewed the breadth of efforts to treat CNS symptoms he saw at 2020 WORLDSymposium™, the leading LD conference[4].

At AVROBIO, we are pursuing lentiviral gene therapy, an approach whereby hematopoietic stem cells (HSCs) are genetically modified. Lentiviral gene therapy is unique in its potential to impact symptoms from head to toe via all the nucleated components of blood, as well as the microglia in the CNS. Microglia are multi-functional and widespread throughout neuronal tissue and offer a compelling avenue to investigate the potential for their own functional correction and/or therapeutic enzyme delivery after lentiviral transduction of HSCs. Others in the industry also use this modality; across the field, transformative efficacy from a one-time treatment with lentiviral gene therapy has been demonstrated across a growing list of indications and clinical trials[5].

Our investigational gene therapies are designed to combine optimized lentiviral vectors and personalized busulfan conditioning deploying state-of-the-art precision dosing. Busulfan is an established conditioning agent in the lentiviral gene therapy field[6] – and it is key to the “head-to-toe” benefits we hope to demonstrate in our therapies.

Indeed, after more than 70 years of clinical use, principally in blood cancers, human data now show that busulfan is finding a new career – as a shepherd to the CNS for the descendants of transplanted HSCs, in particular the monocytes that become microglia cells[6] (See Box 1). We are leveraging this ‘shepherding’ facility of busulfan in our own clinical programs and hope to expand in the future through continued innovation around this remarkable drug, as I recap below (Box 1).

|Box 1| Defining microglia: A primary immune cell within the CNS, and a type of macrophage. Neurologists tend to only use the term ‘microglia’ when the cell in question is derived from primitive macrophages in the yolk sac, rather than from an HSC in the bone marrow. For most intents and purposes, however, referring to them as resident macrophages, microglia-like cells or microglia is interchangeable in relation to their overall phenotype and function.

From team player to solo operator: a new role for busulfan showcases special talents

Busulfan is a clinically validated conditioning agent that has been administered as a single agent in a single cycle to hundreds of patients treated with investigational lentiviral gene therapies[7]. A therapeutic alkylating agent, busulfan has conventionally been used as part of a cocktail of drugs for treating blood cancers.

But emerging data support busulfan as a potentially powerful tool for brain conditioning. This novel feature is due to the low level of busulfan binding to proteins in the blood; as a small molecule unencumbered by larger proteins, it can readily cross the blood–brain barrier[8].

We believe that long-term engraftment of HSC-derived “daughter” cells following gene therapy may lead to the reconstitution of a microglial network, where the new gene-modified microglia have the potential to be functionally corrected and/or drive cross-correction of neuronal tissue via protein secretion and uptake. The key questions leading up to such a potentially outcome: Does busulfan enable genetically modified microglia to engraft in the CNS? And once there, do the microglia produce a functional enzyme? There are now early human data providing insight into these questions, drawing from clinical trials in the devastating disease metachromatic leukodystrophy (MLD), which has led the way in this area in many respects[6],[5].

An MLD clinical story confirms a role for busulfan in addressing
CNS disease

A recent study of two patients who had received allogeneic hematopoietic stem cell (HSC) transplants for MLD with the use of busulfan showed donor-derived macrophages distributed throughout the entire white matter[12]. This collaboration is intended to deliver a novel nanoparticle-immunoassay kit compatible with automated analytical devices commonly used at hospitals and clinics. It is designed to enable much faster, on-site analysis of busulfan metabolism – minutes as opposed to hours. We hope this will greatly expand access to personalized busulfan conditioning deploying state-of-the-art precision dosing, for use not just in the field of lentiviral gene therapy, but across a wide range of HSC transplants for conditions ranging from hematological cancers to autoimmune disease.

It has been incredibly exciting to see the emergence of a technology that can work both sides of the blood–brain barrier and potentially halt, or potentially in some instances reverse, CNS and systemic changes across a range of devastating diseases. Busulfan’s role in that process, after decades of unrelated use in hematological oncology, is a reminder that established drugs can often have new uses, and that biology is a strange and wonderful thing. Ultimately, there’s increased potential for busulfan to serve as a shepherd of gene-modified cells to impact CNS diseases, and AVROBIO is excited to have the opportunity to be a leader in this next frontier for lentiviral gene therapy.

Originally published by Cell & Gene

Chris Mason, M.D., Ph.D. is the Chief Scientific Officer of AVROBIO, Inc.

AVROBIO is currently conducting clinical trials to evaluate the safety and efficacy of its investigational lentiviral gene therapies. None of these investigational gene therapies has been approved by the U.S. Food and Drug Administration or any other regulatory agency. For more information, go to www.avrobio.com.