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UCSF s Frank Szoka On Solving the Nucleic Acid Drug Delivery Problem


At A Glance

Name: Frank Szoka

Position: Professor, biopharmaceutical sciences and pharmaceutical chemistry, University of California, San Francisco

Background: Associate professor, pharmacy and pharmaceutical chemistry, UCSF — 1985-1991; Assistant professor, pharmacy and pharmaceutical chemistry, UCSF — 1980-1985; Assistant professor, physiology, Tufts Medical School — 1979-1980; Cancer research scientist, Roswell Park Cancer Institute — 1978-1979; Postdoc, RPMI — 1976-1978; Captain, US Army/director, clinical chemistry, Walson Army Hospital — 1969-1971; PhD, biochemistry, State University of New York, Buffalo — 1976; MS, microbiology, University of Maryland — 1971

Though he always has kept at least one foot in academia, Frank Szoka’s drug-delivery experience extends into industry, as well: He co-founded Sequus Pharmaceuticals, a drug company that developed cancer treatments using novel drug-delivery technologies and was later acquired by Alza (which itself was acquired by Johnson & Johnson). He also founded GeneMedicine, a gene therapy products developer that was acquired by Valentis.

Now extending his research into RNAi, Szoka recently took time to speak with RNAi News about his work.

Could you give an overview of what you do with delivery technologies?

Our goal is to use peptides, lipids, and polymers to deliver nucleic acid drugs and other large molecules into cells. So we synthesize new molecules, and then try to create them so that they self-assemble into some type of carrier that’s smaller than 100 nanometers. Most of our work focuses on either anticancer drug delivery or anticancer gene delivery.

What sort of nucleic acids have you traditionally been working with?

Traditionally, we have been working with genes and antisense oligonucleoides. …

Your readers certainly know that any nucleic acid drug is difficult to deliver, and the biggest problem that such agents have is their molecular weight. So crossing from outside the cell to inside the cell is the barrier. Then there are a number of other barriers along the way — [the agents] may be rapidly eliminated if they’re too small, they may be rapidly metabolized if the backbone isn’t correct — but the major difficulty is getting the molecules from a systemic injection into the site where you want them, and having them cross the cell membrane to get into the target cell.

This is the challenge for everybody who’s doing nucleic acid drug delivery. The problem is not too much more difficult for large genes than it is for small oligos. So what most people have been trying is to either devise a carrier that will help the molecule negotiate across the cell membrane, or to place some sort of small ligand or other effector onto the nucleic acid if its an siRNA or oligonucleotide. Again, [they’re trying to] exploit some aspect of transport or self-association with the cell to get the nucleic acid drug inside.

How long have you been working with RNAi molecules?

We’ve only been working with RNAi for about two months.

How did you get started?

We were approached by a number of different groups who were interested in delivery, and having worked in antisense oligonucleotides for a substantial period of time in the late 80s and early 90s, many of the things we had done with those molecules were transferable to siRNA. And the siRNA has a much more reproducible spectrum of activity, so they’re a little easier … to work with because you can see an effect in a more producable, sequence-specific fashion. Whereas with oligonucleotides, particularly thioates, there were a number of off-target effects that were caused by other biology and chemistry that was going on in the end.

So our entry back into small nucleic-acid drugs just came about because so many people called us and said, “Can you deliver this molecule?”

Are these academic groups that are looking for your help or industry?

Both academic groups and industry.

So your lab has collaborations with people in the RNAi therapeutics field?

Not really. We work with them, [but] at this point it’s premature to call them collaborations.

Can you talk about the work you’re doing with [these] partners?

No, I can’t talk about anything that’s done with non-academic groups.

You are working with some academic groups though?

Yeah. In-house we’re working with academic groups to deliver oligos to inhibit prion formation. We work with the [Stanley] Prusiner group [at UCSF]. … He’s been interested in using antisense molecules to interfere with the infectious process of prions.

As part of that, we had worked with him using dendrimers, which turned out … to have an anti-prion effect … in their own right. More recently, we’ve been working with him on phosphorothioates, and now there’s a move to examine siRNAs [with his group].

Are there other RNAi projects you have going on there?

The other things are involved with down regulation of certain signaling molecules in cancer.

Can you talk about those?

I’m not the lead investigator. I can’t.

Do you still do work with antisense?

We still do work with antisense, but we use it mainly as an indicator of cytoplasmic delivery. So with a molecule like a phosphorothioate antisense molecule, if you can deliver it into the cytoplasm, it rapidly accumulates into the nucleus. So it’s a very sensitive assay for successful cytoplasmic delivery.

You spoke about the things that make delivery challenging for nucleic acid drugs. Are there things particular to siRNAs that make them a different problem, … or is this a continuation of what you saw with antisense?

A continuation of the same things you observe with antisense.

What is the number one problem in dealing with delivery?

The number one problem is getting a sufficient amount of the dose across the plasma membrane into the cytoplasm.

The second problem is reaching a significant number of the cells that you would like to treat with the molecule. So if you have a solid tumor, it’s very difficult to reach every single tumor cell in that tumor. And as the size of the molecule increases, the more difficult it becomes. Our approach is to build something around the siRNA, a liposome or a dendrimer.

What approach seems most promising at this point?

I don’t have the slightest idea. I have to be perfectly honest with you; I think it’s a hard problem. You tell me what the target is and what disease you want to treat and how frequently you want to treat that disease, then maybe I can guess what might be a more promising approach.

But if you think of it, siRNA down regulates some type of protein. So if you want to do this on a chronic fashion — and the best effects [of siRNA] probably don’t extend more than four or five days — that means you’re talking about a once-weekly injection. If you put yourself into the category of a once-weekly intravenous injection, if the target is not … some sort of special compartment where you can keep you molecule in there at high concentrations for long times [like] … the eye, … you’re talking about chronic injections.

That is the major problem. It’s in the nature of the nucleic acid drug.

That being the case, what’s your take on the RNAi therapeutics field … in terms of systemic types of drugs?

I think they’re following in the footsteps of the oligonucleotide delivery field, and what has been the success there? There hasn’t been any oligonucleotides that work by an antisense effect used for a disease that has a big market. As far as I know, there’s one antisense oligonucleotide, I’ll call it a phosphorothioate oligonucleotide, that’s approved. But … it works by a non-specific mechanism.

So, I would say that for the types of applications that I’m hearing about [for] siRNA, I think you’re recapitulating 15 years worth of drug development work from companies like Isis [Pharmaceuticals].

Where are antisense oligonucleotides starting to maybe have some effect? As vaccine adjuvants working through Toll receptors. If you look at companies like Coley [Pharmaceutical] or Dynavax [Technologies], these companies are using antisense oligonucleotides to manipulate the immune system. By doing so, they have changed the problem, so they don’t have to deliver the molecule weekly or monthly; they deliver it a few times and they can boost the immune system.

I don’t want to rain on anybody’s parade, but the nature of the pharmacology of these molecules creates a very difficult delivery problem.

Nonetheless, you’re still working on it, too.

I’m working on it because I work on drug delivery. It’s not like I’ve selected my field. I’d have to change my field and work on something else.

The approaches that we’ve taken … for the past 15 years are to make bio-responsive systems: systems that are pH-sensitive or reduction-sensitive, and either undergo a phase change — they go from something that is a liposome to something that releases its contents and fuses to cells — or something that explodes in the endosome to attempt to penetrate the cell in that fashion. …

I think in the long term these things will be deliverable. Particularly the siRNAs, because they actually have pharmacology that seems to be reproducible and at nanomolar levels, and that’s a big change from antisense oligonucleotides. Even the best [antisense oligos] probably were 100 nanmolar, and most of them were 3, 4, 500 nanomolar. But some of these siRNAs seem to be getting down to single nanomolar levels of activity, and that’s very encouraging.

The less you have to deliver, the easier it is.

What is your take, if you have one, on the gene therapy kind of approach in RNAi?

I think it combines the difficult of delivering large molecules with the problems of gene therapy. … Everybody wants to stay away from it because there’s a patient registry requirement, so you have to follow patients for 15 years … for clinical trials. So the cost of doing such clinical trials has gotten more expensive.

[Additionally,] the efficiency of most vectors for gene therapy is not so hot, even the viral vectors. And with most of the viral vectors you run into immunological problems on re-administration. So going to gene therapy is a creative academic way to do siRNA, but it doesn’t hold out the prospects for a more rapid to market product. It might actually be slower.

Speaking of the commercial market, are you ever planning on getting back into industry?

Right now, my life is a little bit complicated. I have a couple of unfinished things that I have to resolve before I’d get back into commercial drug delivery.

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