Boosting Cancer Therapy: A New Trick for Astatine-211

Hey there! Let’s chat about something pretty exciting happening in the world of fighting cancer. We’re talking about a cool approach called Targeted Alpha Therapy (TAT). Think of it like sending tiny, super-powerful snipers directly to cancer cells to take them out, leaving the healthy guys alone as much as possible. And one of the star players we’ve got our eye on for this job is a radioactive element called Astatine-211, or ²¹¹At for short.

Now, ²¹¹At is fantastic for a bunch of reasons. Unlike some other radioactive isotopes that are tricky to get or have complicated decay chains, ²¹¹At can be made right here in a medium-energy cyclotron – basically a particle accelerator – using something stable and pretty common, Bismuth-209. Plus, it emits just one high-energy alpha particle, which delivers a potent punch over a very short distance, perfect for killing cancer cells nearby without causing too much damage further away. And its half-life is just 7.2 hours, which is great because it means less lingering radioactivity in the patient after treatment.

But here’s the catch, the little bit of drama in our story: getting ²¹¹At to the cancer cells usually involves attaching it to a targeting molecule, often by forming a chemical bond between the astatine atom and a carbon atom on a ring-like structure (we chemists call this an “aryl” group). The most common way to do this uses something called [²¹¹At]astatobenzoate, or [²¹¹At]SAB. It’s been used in lots of studies, even in clinical trials. The problem? This bond, the carbon-astatine (C-At) bond, can be a bit fragile once it’s inside the body.

The Problem with Astatine’s Bond

Imagine our targeted molecule as a delivery truck carrying the ²¹¹At sniper. When this truck gets inside a cancer cell, it might end up in places like lysosomes (sort of the cell’s recycling or garbage disposal units, which can be acidic and oxidative) or encounter enzymes like CYP450, which are great at breaking things down, including chemical bonds.

What we’ve seen, both in early lab tests and sadly, confirmed in those clinical trials, is that this C-At bond can break. When it breaks, the ²¹¹At is released from its delivery truck. And where does it go? Well, it tends to wander off to healthy organs like the thyroid, stomach, spleen, and lungs. This is obviously not what we want! It reduces the dose going to the tumor and raises safety concerns because healthy tissues are getting hit.

Previous work by other brilliant minds has hinted that this bond breaking, or “deastatination,” is often triggered by those oxidative conditions we mentioned. They even showed that astatine compounds seem to be easier to oxidize than their iodine cousins, and once oxidized, the C-At bond is significantly weaker than a C-I bond. So, clearly, oxidation is a major culprit.

Our Bright Idea: Ortho-Functionalization

Building on this understanding, we had a thought: what if we could protect that vulnerable C-At bond? What if we put some chemical “bodyguards” right next to the astatine atom on that aryl ring? We call this “ortho-functionalization” – adding groups in the positions immediately adjacent to where the astatine is attached.

Our hypothesis was that adding benzyl alcohol groups in these ortho positions might shield the C-At bond, making it more resistant to oxidative attack. We decided to investigate molecules with either one or two of these benzyl alcohol groups right next door to the astatine. We also compared them to a reference compound, [²¹¹At]AEB, which is a good stand-in for the kind of bond found in the less stable [²¹¹At]SAB derivatives.

Putting it to the Test (Synthesis e Radiolabeling)

First things first, we had to make these molecules. It involved a few steps in the lab, starting from readily available materials. We used a method called nucleophilic ²¹¹At-labeling with aryliodonium salts, which is known to work well for making these ortho-substituted compounds. We managed to successfully synthesize the precursors for our mono- and di-functionalized compounds, as well as our reference.

Then came the exciting part: getting the ²¹¹At onto them! We reduced the ²¹¹At to its reactive form and hooked it onto our precursors. We were pretty pleased to see that we got high radiochemical yields – meaning a good amount of the ²¹¹At ended up attached to our desired molecules, even with those benzyl alcohol groups nearby. After a bit of purification to make sure we had the pure product, we were ready for the crucial stability tests.

The Acid Test: Oxidative Stability

Our first test was designed to mimic the acidic, oxidative environment found inside those cellular lysosomes. We incubated our ²¹¹At-labeled compounds in a solution containing potassium permanganate (a strong oxidizer) at a slightly acidic pH. We then checked over time using a technique called HPLC to see if the ²¹¹At was still attached or if it had broken off.

And wow, the results were clear! Our reference compound, [²¹¹At]AEB, showed significant degradation, with lots of free ²¹¹At detected after just 20 hours. This matched what others had seen before and confirmed our starting point – this bond *is* unstable under these conditions.

But our ortho-functionalized compounds told a completely different story. The compound with *two* benzyl alcohol groups (compound 3) showed virtually *no* free ²¹¹At release! It was rock solid. The compound with one benzyl alcohol group (compound 2) also showed significantly improved stability compared to the reference, though not quite as good as the one with two groups. This clearly showed a gradual effect – the more benzyl alcohol bodyguards, the better the protection against oxidation. Interestingly, for compound 3, while the C-At bond stayed intact, the benzyl alcohol groups themselves got oxidized a bit by the permanganate, turning into carboxylic acids. It seems the bodyguard took the hit, protecting the main target!

Tackling Metabolism: Microsome Stability

Next, we wanted to see how our compounds would fare against the body’s metabolic enzymes, specifically the CYP450 enzymes found in liver microsomes. These enzymes are known to cause dehalogenation (breaking carbon-halogen bonds, including C-At bonds). We set up an assay using rat and human liver microsomes to see how stable our compounds were.

Again, the trend was the same. [²¹¹At]AEB (our reference) showed rapid deastatination, releasing free ²¹¹At in both rat and human microsomes. The rest of the activity turned into other metabolic products, likely from the enzymes working on other parts of the molecule.

In stark contrast, the difunctionalized compound (compound 3) was incredibly stable. We saw practically *no* release of free ²¹¹At in either rat or human microsomes. It seems those two benzyl alcohol groups did a fantastic job of protecting the C-At bond from enzymatic attack. No other metabolic products were seen for this compound either, suggesting the enzymes couldn’t easily find a spot to work on it. The monofunctionalized compound (compound 2) was again in the middle, showing better stability than the reference but less than compound 3.

We did notice that dehalogenation was generally faster in rat microsomes than in human ones, which is interesting but doesn’t change the overall picture of stability provided by the ortho groups. We also checked if the amide group in our reference compound was somehow making it *less* stable than a simple astato-benzene. It wasn’t – astato-benzene showed a similar instability profile, confirming that the instability of the reference is inherent to that type of C-At bond under these conditions, not just because of the amide.

Why Does This Work? Unpacking the Mechanism

So, why do these benzyl alcohol groups provide such great protection? We think there are a few things going on.

* Steric Hindrance: The bulky benzyl alcohol groups right next to the astatine might simply get in the way, making it harder for large enzymes like CYP450 to get close enough to the C-At bond to break it.
* Hydrophilicity: The alcohol groups make the molecule a bit more water-loving, which could also influence how enzymes interact with it.
* Intramolecular Interaction: It’s possible that the alcohol groups might form a temporary internal bond or interaction with the astatine atom itself, perhaps forming a transient ring-like structure (like an arylakoxyastatinane). This could shield the astatine and the C-At bond from external attackers. We couldn’t directly *see* this intermediate, but it’s a plausible explanation for the protection seen even in the purely chemical oxidation test where enzymes aren’t involved.
* Halogen-Bond Protection: Another recent idea in the field is that astatine can interact with biological molecules (like certain enzymes) through something called a “halogen bond,” and this interaction might trigger the C-At bond to break. The hydroxyl groups on our benzyl alcohols are great at forming hydrogen bonds, and they might compete with astatine’s ability to form halogen bonds, effectively distracting or blocking those unwanted interactions.

What’s Next? Looking Ahead

Our results are really encouraging! They show that ortho-functionalization with benzyl alcohol groups is a very promising strategy to make ²¹¹At-labeled aryl compounds much more stable against oxidative degradation, which is a major cause of instability in vivo.

We were also excited to see that another research group published work during the revision of our article showing improved *in vivo* stability in mice using a very similar astato-benzene compound also substituted with benzyl alcohol groups in the ortho positions. Their results showed less uptake of ²¹¹At in healthy organs like the stomach and spleen, which is a strong sign that the C-At bond was staying intact in the body. This independent finding really supports our conclusions and the potential of this approach.

Besides protecting against oxidation, these groups might also help protect against that halogen-bond mediated dehalogenation mechanism we just mentioned. Plus, adding these groups makes the molecule more polar, which could be beneficial for attaching ²¹¹At to smaller targeting molecules like peptides without messing up their ability to find the tumor.

Of course, this is just the beginning. We need to explore how this strategy works with different types of aryl compounds and other ortho substituents. The next big step is to take these stabilized compounds and attach them to actual targeting molecules (like antibodies or peptides that seek out cancer cells) and then test them in vivo to see how they perform in a living system.

We’re actively working on developing these functionalized derivatives and testing them out. We’re optimistic that this approach will pave the way for designing new ²¹¹At-radiopharmaceuticals with significantly improved stability, ultimately leading to safer and more effective targeted alpha therapy for cancer patients. It’s a complex challenge, but seeing results like these makes all the hard work worthwhile!

Source: Springer