Phase I dose-escalation trial combining CRLX101 (nanoparticle camptothecin) with olaparib using a 48-hour gapped schedule in 24 patients with advanced solid tumors to determine MTD and assess pharmacokinetics, pharmacodynamics, safety, and preliminary efficacy.
0:00Welcome to Base by Base, the papercast that brings genomics to you wherever you are. Thanks for listening, and don't forget to follow and rate us in your podcast app. So imagine you have this incredibly tough industrial cleaning job.
0:13Right, like something really stubborn. Exactly. And you have 2 incredibly powerful chemical solvents. Okay. Now, individually, they do an okay job on the grime. But you realize that if you mix them together, their combined reaction destroys the grime flawlessly.
0:29I feel like there's a catch here. There's a massive catch. There was one big problem. The moment you mix them, the chemical reaction is so violently intense that it completely melts the bucket you were using.
0:40Ah, yeah, that that would be an issue. Right. So if you're listening to this, you can probably guess that in the world of oncology, the grime is an advanced solid tumor. The chemicals are 2 very specific cancer drugs.
0:52And the bucket, the thing that basically keeps melting, is the patient's entire bone marrow system. Yeah, that is a fantastic analogy. And, you know, today we celebrate the work of the oncology researchers, behind a remarkable 2025 paper in nature communications, who have advanced our understanding of the DNA damage response.
1:12Because it's a huge step forward, right? Our mission for this deep dive is to explore how these researchers essentially, well, hacked the timing and delivery of existing cancer drugs. Yeah, to unlock a combination therapy that was historically considered way, way too toxic for human use.
1:28What's fascinating here is that their work is this brilliant pivot from from what chemicals we use to how and when we deploy them inside the human body. Because finding that balance between aggressively killing a tumor and actually sparing the patient's healthy tissue is, uh, it's basically the ultimate puzzle in cancer research.
1:45It really is. the whole game. Okay, let's unpack this. If the bucket melts, the moment these drugs meet in the bloodstream. What is the actual biological explosion happening at the cellular level? I mean, why did doctors even want to combine these 2 specific drugs in the 1st place?
1:59Right. So to understand the combination, we have to look at the cellular machinery that keeps us alive. Specifically the DNA damage response or DDR. The DDR. Yeah. Every time a single cell divides, it has to unzip and copy its entire DNA sequence.
2:14It's this highly complex physical process, and, you know, naturally mechanical mistakes happen. Like typos in a document. Exactly. The DDR is the cell's built-in repair crew that detects and fixes those typos.
2:27Now, cancer cells are notoriously unstable. They are dividing chaotically and constantly. Meaning they're making a massive amount of copying errors. Huge amounts. Because of that, they rely heavily on this DDR machinery just to survive their own reckless growth.
2:41Right. If you are dividing that fast, your spell checker has to be working overtime. That is the perfect way to look at it. And that brings us to the 2 drugs. So the 1st is a Topois Mari's one inhibitor, or a TOP one inhibitor.
2:54To Boyzema race one is an enzyme that acts like a molecular swivel. When DNA unzips to be copied, it gets tangled and twisted. So this enzyme temporarily snips one strand of the DNA to relieve the tension, lets it untwist, and then reseals it.
3:09It's like untangling a phone cord. Yes. But the inhibitor drug-like camp to Thasin sabotages this process. It lets the enzyme make the snip, but prevents it from resealing the cut. It intentionally leaves a single strand break in the DNA.
3:24Okay, so drug number one goes in and deliberately leaves the tumor cell's DNA physically snapped open during replication. But a single strand break isn't always fatal to a cell, right? I mean, wouldn't the cell spell checker just call in the repair crew to fix the snip?
3:38It would, which is exactly where drug number 2 enters the picture. A PRP inhibitor, like OlaParim. Ah, okay. PROP is a protein that acts as the cell's 1st responder to single strand DNA breaks. And originally, scientists thought PRP inhibitors just blocked this protein from doing its repair job.
3:54The stop sign. Right, but they actually do something far more devious. They physically trapped the PARP protein right on top of the broken DNA strand. It creates this rigid roadblock. Oh, wow. So the DNA strand is snipped open, and the repair mechanic is essentially glued directly onto the broken machinery.
4:12Yes. And the result is catastrophic for the cell. This is called a TOP one DNA cleavage complex. You have a broken strand with a protein trapped on it. And then what happens? Well, when the cells replication machinery comes barreling down the DNA track to copy the genetic code, it slams into this roadblock.
4:32Like a train hitting a boulder. Exactly. The reputation fork collapses. What started as a manageable single strand snips shatters into a highly toxic double strand brake. Oh man. Yeah, the DNA is completely severed, the tumor cell undergoes massive unrepairable damage, and just initiates self-destruction.
4:48Wow. From a purely theoretical standpoint. That sounds like a flawless trap. I mean, you break the tumor's DNA, and then you weaponize its own biological repair kit against it. It's brilliant on paper.
4:59But circling back to our melting bucket scenario. If this trap is so perfect, what was the historical roadblock, like when doctors tried this in previous clinical trials, what actually happened? They hit a wall, a wall called dose limiting toxicity, specifically severe milus oppression.
5:17Milus oppression, that's the bone marrow issue, right? Right. When you give these 2 drugs systemically, they don't just find the tumor. They go everywhere. And what tissue in the adult human body is constantly rapidly dividing, much like a tumor?
5:30The bone marrow. Because it's constantly pumping out, what, red blood cells, white blood cells, platelets? Exactly. Because those bone marrow cells are rapidly dividing, their DNA is constantly unzipping, making them incredibly vulnerable to this exact same DNA damaged trap.
5:45Oh I see. Yeah, when previous trials gave patients a TOP one inhibitor and a PRP inhibitor at the same time, the combination essentially wiped out the patient's immune systems and blood supply. That's devastating.
5:57The collateral damage was just too high. Way too high. To keep the patients alive, doctors had to drastically reduce the doses. How drastically are we talking here? Down to just 3% of the standard PIRP inhibitor dose and roughly 40% of the standard TOP one inhibitor dose.
6:153%. Wait, at that point, you aren't even administering a therapy. That's practically a homeopathic dose. Yeah, and unsurprisingly, at those reduced doses, the treatment became completely ineffective against the tumors.
6:27The doctors were trapped in this biological paradox. Damned if you do, damned if you don't. Exactly. If you give enough of the combined drugs to kill the tumor, you destroy the marrow. If you protect the marrow by lowering the dose, you let the tumor grow.
6:41You know, if you're listening to this and wondering why on Earth researchers didn't just throw in the towel, I mean, it's a valid question. If a chemical combination burns a hole through the floor, the logical step is to stop using it and look for a completely different chemical.
6:55Why stubbornly stick with these 2 drugs? Because the synergy of that double strand DNA brake was undeniable. The mechanism was just too potent to abandon. They knew it worked. Right. The cancer cells were incredibly sensitive to it in the lab.
7:09The researchers behind the new study realized the problem wasn't the biology of the drugs, the problem was the logistics of how they were being delivered. Okay. They needed to keep the chemicals from meeting in the bone marrow.
7:20Which brings us to the two prong solution they developed. Let's start with the 1st part of the logistics, which is the delivery mechanism. How do you get a drug to ignore the bone marrow and only go to the tumor?
7:31They utilized a drug called CRLX 101. Now, this isn't your standard free floating TOP one inhibitor. It is a nanoparticle formulation of Camp Defacin. They package the active drug inside a microscopic polymer carrier.
7:47Ah, okay. Think of it like a slow release fertilizer capsule you put in your garden. If you pour liquid fertilizer directly on a delicate plant, it might burn the roots. But a capsule protects the plant and only releases the nutrients under very specific conditions.
8:01But how does this microscopic capsule know to accumulate in a tumor? It takes advantage of something called the enhanced permeability and retention effect or EPR? EPR, okay. Yeah. When tumors grow rapidly, they are desperate for a blood supply, so they hastily build their own blood vessels.
8:18But these tumor blood vessels are poorly constructed. Shoddy workmanship. Exactly. They are structurally shoddy and highly leaky, unlike the tight, well formed blood vessels in healthy tissue like the bone marrow.
8:30So as the nanoparticles are circulating in the bloodstream, They safely bounce past the tight walls of the healthy bone marrow, but when they reach the leaky plumbing of the tumor, they just slip right through the cracks.
8:41Yes, and because tumors also have core lymphatic drainage, once the nanoparticles fall through those cracks, they get trapped there. They can't get back out. Right. They accumulate deep inside the tumor tissue, slowly dissolving and releasing the TOP one inhibitor directly into the cancer cells.
8:59It essentially localizes the 1st half of our DNA trap exactly where we want it. Okay, so that solves the delivery problem. But if you administer the PREP inhibitor at the same time, wouldn't you still amplify whatever small amount of the TOP1 drug happen to be circulating in the healthy marrow?
9:16You would. And that is why the nanoparticle alone wasn't enough. The 2nd half of the solution is a strict timing protocol called gapped scheduling. Gapped scheduling. Yeah, the researchers needed to figure out exactly how long the bone marrow took to process and clear any minimal exposure to the 1st drug.
9:33Interestingly, to figure this out, they relied on pre-clinical models using rats rather than the traditional mouse models. Why does the specific rodent matter for testing bone marrow toxicity? It matters deeply because mice are remarkably resilient when it comes to blood cell recovery.
9:50A mouse's hematopoietic stem cells cycle and recover from stress far faster than human cells do. Oh I see. Yeah, if you test bone marrow toxicity in a mouse, you will get a false sense of security. Rat bone marrow, however, has DNA repair dynamics and recovery timelines that much more closely mimic human marrow.
10:08That's fascinating. So the rats gave them a more realistic window of human vulnerability. What was the magic number they discovered? 48 hours. The rap model showed that if you introduce a strict 48 hour delay, you can mitigate the catastrophic marrow toxicity.
10:23Okay, so how does that look in practice? So the clinical protocol became this. Administer the CRLX 101 nanoparticle on day one. Let it circulate, let it fall through those leaky tumor vessels, and let the bone marrow process, and clear its minimal exposure.
10:38Like, clear out. Right. Wait entirely through day 2. Then, on day 3, start administering the PRRP inhibitor, Ola perip. Okay, so by gapping the schedule. You give the healthy bone marrow time to clear the area and reset.
10:51But because the nanoparticle is physically stuck inside the tumor slowly releasing its payload over time, the cancer cells are still fully primed and vulnerable when the PRP inhibitor finally arise on day 3 to spring the trap.
11:04Exactly. The mechanism is elegantly simple once you map out the timelines, but, you know, the true test is whether this logic holds up in human biology. Right. Let's transition to that real world application.
11:14They move this Trojan horse and 48 hour timer strategy into a phasei clinical trial with 24 human patients. Now, phase trials were primarily about finding the maximum tolerated dose. Yes, safety first.
11:26Given that previous trials couldn't get past 3% for the PRP inhibitor, how high were they able to push the doses with this new logistical approach? The results were staggering. At their target dose, which they call dose level 4R, they administered 12 milligrams per square meter of the CRLX 101 and 250 milligrams of the old purrib twice daily.
11:48And what does that mean in percentages? Using this gap schedule, they safely achieved roughly 80% of the single agent maximum doses for both drugs simultaneously. Wow. Going from 3% up to 80% is a massive expansion of the therapeutic window.
12:02It's a total game changer. Here's where it gets really interesting, though. How do we know the drugs were actually interacting inside the body? If the nanoparticle takes 48 hours to settle, how do doctors verify the trap was actually set and triggered at a cellular level without, you know, cutting the patient open every 2 days to check the tumor?
12:21That's a great question. They needed a non-invasive way to measure pharmacodynamics to prove the drugs were causing the intended DNA double strand brakes. So they looked for a highly specific biomarker called Gamma H2AX.
12:34Gamma H2AX. What exactly is that? It is a phosphoryulated histone protein. Basically, when DNA suffers a severe double strand break. The cells machinery rapidly tags the surrounding area with this protein.
12:49Like a biochemical flare. Precisely. It marks the exact site of the catastrophic damage. To find these flares without performing invasive organ biopsies on these highly vulnerable patients, the researchers did something incredibly clever, they plucked hairs from the patient's scalps.
13:04Wait, scalp hair. How does a hair follicle tell you about DNA damage from a cancer drug? Because hair bulbs contain keratinocytes, which are actively rapidly dividing cells. Oh, just like the tumor in the bone marrow.
13:16Exactly. They are highly proliferative, making them a fantastic surrogate tissue to observe how drugs impact DNA replication in real time. They plucked hair follicles on day one for a baseline. Then again on day three, 48 hours after the nanoparticle.
13:30And what do they see? They saw a clear increase in the gamma H2AX flares, proving the TOP1 inhibitor was successfully causing the initial single strand breaks. Okay, so the 1st half of the trap was set.
13:40Yes. Then they plucked hairs again on day four, 24 hours after starting the PRP inhibitor. The biochemical flares spiked even higher. Wow. Yeah, the DNA damage signal amplified significantly, providing definitive molecular proof that the 12 punch was actively synergizing in human tissue.
14:01Okay, but finding a chemical flare and a hair follicle proves the biological mechanism is sound across the body. But firm codynamics on the scalp doesn't automatically equal a shrinking tumor in the pancreas of the lungs, right?
14:15No, it doesn't. The micro environment of a solid tumor, the acidity, the blood flow, is vastly different from a hair follicle. When they looked at the actual clinical outcomes for these 24 patients, did this gapped therapy actually halt the cancer?
14:29That is the critical limitation of surrogate tissues, which the authors openly acknowledge. A hair follicle proves the drugs are working as intended, but only patient scans can prove efficacy. And keeping in mind that these were patients with highly advanced, solid tumors who had already exhausted standard therapies.
14:45The clinical data was highly encouraging. Yeah, out of 19 patients who were evaluable for a response, 2 achieved partial responses, meaning the tumors shrank significantly. 6 more achieved stable disease, meaning their tumor growth was halted.
14:59I mean, in a population that has failed multiple lines of prior chemotherapy, stabilizing the disease is a profound victory. It translates directly to time. The median overall survival was 6.06 months, but the individual patient stories are even more striking.
15:15Like what? There was a patient with relapse mixofibrousarcoma who had a deep sustained response lasting over 7 months. Another patient battling heavily pre-treated colangiocarcinoma, which is a notoriously aggressive bioduct cancer, maintained a partial response for 6 months.
15:33That is remarkable time given back to those patients. Now, as we look deeper into the data, there is a fascinating genetic twist to who actually responded to this treatment. Usually, a patient needs a very specific genetic profile to even be offered a PRP inhibitor, right?
15:46Traditionally, yes. This is a concept called synthetic lethality. Imagine a cancer cell has 2 different pathways to repair its DNA pathway A and pathway B. Okay, tracking. If a drug blocks pathway A, the cell just uses pathway B and survives.
16:00But some patients have genetic mutations, like the well-known BRCA mutations in breast and ovarian cancer, meaning their tumor cells are born with pathway A already broken. This is called a homologous recombination deficiency or HRD.
16:13So their natural spell checker is already fundamentally compromised. Exactly. When you give a PRP inhibitor to a patient with a BRCA mutation, you are blocking pathway B. Because pathway A was already genetically broken.
16:27The cell has 0 options left and it dies. That is synthetic lethality. And indeed, the mix of fiber sarcoma patient who responded so beautifully in this trial, had a mutation in a gene called PLB2, which functions very similarly to a BRCA mutation.
16:41Okay, that makes perfect logical sense. The treatment worked because the patient had the expected genetic vulnerability. So what does this all mean for the other patients? What was the twist? The twist was that patients without any obvious genetic defects in their DNA or repair machinery also responded to this combination.
16:58Wait really? Yeah. When the researchers analyze the broader genetics of the responders versus the non-responders, they found no statistically significant correlation regarding who had these predicted loss of function variants.
17:10Having the genetic vulnerability didn't guarantee a response, and importantly, lacking the mutation didn't rule out a response. But if the cancer cells had a perfectly healthy pathway, A, how did the treatment still kill them?
17:23The prevailing theory is that the physical mechanism of trapping the TOP1 enzyme and the PRP protein directly onto the DNA strands creates such an overwhelming amount of catastrophic double strand brakes that it entirely exhausts the cells repair capabilities.
17:40It just overloads the system. Right. It doesn't matter if the cell has a healthy spell checker. You are essentially throwing a wrench into the printing press itself. The gapped combination creates a synthetic vulnerability on the fly, overpowering the tumor regardless of its underlying genetic background.
17:55So what does this all mean for the future of oncology? If specific mutations like BRCA or HRD scores aren't the ultimate gatekeepers for this combination therapy, this could broaden the horizon for who is eligible for these treatments.
18:09It entirely blows the doors open. The next step is larger biomarker enriched phase 2 trials to refine exactly how we predict responses. We might need to look at the real-time expression of certain proteins in a tumor rather than relying on static genetic mutations.
18:25That makes sense. It suggests that highly potent gap scheduled therapies could become a broad spectrum approach for tumors that exhibit high replication stress. Let's distill all of this into a clear takeaway.
18:36What we've explored today is a masterclass in medical problem solving by combining smart delivery using a microscopic nanoparticle that exploits the leaky blood vessels of a tumor, with smart scheduling and forcing a strict 38 hour gap based on the recovery dynamics of rat bone marrow scientists, successfully rescued a highly potent cancer therapy.
18:54They really did. They took a combined treatment that historically had to be dialed down to a useless 3% to prevent marrow toxicity and safely pushed it up to 80%, unlocking a new frontier in DNA damage response therapies.
19:08And if we connect this to the bigger picture, this trial is really just the vanguard of a massive logistical shift in oncology. How so? Well, the concepts of gap schedule and targeted delivery are paving the way for even more sophisticated tools.
19:21The paper highlights the future integration of ADCs, or antibody drug conjugates. ADCs. Right. Instead of a polymer nanoparticle relying on leaky blood vessels, imagine attaching the TOP1 inhibitor directly to an antibody engineered to seek out and bind only to specific proteins on a cancer cell surface.
19:40Like a microscopic heat seeking missile. Precisely. And alongside that, the development of next generation PRP inhibitors. Current drugs inhibit both PRP1 and PR2 proteins. Okay. there a difference? A huge difference.
19:52Emerging research shows that inhibiting PRP is heavily responsible for the toxicity seen in red blood cell development. Oh, so if you leave PRP 2 alone. Exactly. If future trials combine antibody drug conjugates with highly selective PRP1 inhibitors, this gap strategy could become exponentially safer and more devastating to the tumor.
20:12Which leaves us with a truly fascinating thought to mole over. If we can use clever timing, biological gaps, and microscopic delivery vehicles to make impossible drug combinations safe and effective. What other failed historical treatments might be sitting right now in the medical graveyard?
20:29That's the real question. How many other highly toxic melting bucket therapies are just waiting for the right logistical delivery mechanism to bring them back to life? It's amazing to think about. The answers to some of our hardest medical challenges might already be on the shelf, simply waiting for a new strategy of execution.
20:44This episode was based on an open access article under the CCBY 4.0 license. You can find a direct link to the paper and the license in our episode description. If you enjoyed this, follow or subscribe in your podcast app and leave a 5 star rating.
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