Drug innovation is typically framed in terms of action: new targets, new molecules, new mechanisms of action. But some of the most consequential advances in drug treatment today are not solely about what a drug does – they are about how the drug behaves in the body. Surviving long enough in the bloodstream, reaching the right organ, releasing a treatment at the right moment: these are engineering problems, and solving them can be as difficult as finding the drug itself.
A cluster of recent approvals and late-stage trials results makes this visible. The therapies I want to discuss today span different diseases entirely: HIV prevention, bleeding disorders, cardiovascular disease – but share a common thread: Each works in large part because someone solved a delivery problem that was blocking the medicine from reaching its potential.
Lenacapavir: Convenient and Effective HIV Prevention and Treatment
Pre‑exposure prophylaxis (PrEP) is a preventive approach for HIV infection. It involves HIV‑negative individuals taking medication regularly to reduce their risk of acquiring HIV. PrEP uses antiretroviral drugs – the same types of drugs used to treat HIV infection – to prevent infection by blocking viral replication. It is intended for people at increased risk of HIV, such as those with HIV-positive partners, multiple sexual partners, or who share injection equipment.
To be effective, the drug must already be present in the body at an effective level to successfully prevent the establishment of an infection when HIV exposure occurs. The most common approach to PrEP is daily dosing of an antiviral medication. The challenge with the usefulness of PrEP it depends on consistent daily adherence.
Lenacapavir is one of the most consequential recent advances in HIV treatment and and prevention. Its innovation is not just incremental: it introduces a new drug target, a distinct pharmacologic profile, and a fundamentally different dosing paradigm.
Most existing HIV drugs target viral enzymes, such as reverse transcriptase, integrase, or protease. Lenacapavir targets the viral capsid—the protein shell that surrounds and protects the virus’s genetic material. The capsid plays a critical role at multiple stages of the viral life cycle, and disrupting it interferes with HIV replication in several ways.
Its pharmacologic properties are also unusual. Lenacapavir is highly potent, and when administered by subcutaneous injection, it has a long tissue half‑life. As a result, therapeutic drug levels can be maintained for months after a single dose. This enables dosing as infrequently as twice per year – dramatically different that daily oral PrEP.
Clinical trials show that lencapavir provides very high levels of protection against HIV infections and is superior to daily oral PrEP, and eliminates the need for strict daily pill compliance. In that sense, lenacapavir represents a different kind of advance: not just a more effective drug at the molecular level, but a pharmacologic solution to a behavioral limitation. By reducing the need for a daily patient habit, it has the potential to meaningfully expand the impact of HIV prevention. The remaining constraint is no longer scientific, but economic. In the United States, lenacapavir is priced at ~$28,000 per year, an unimaginable cost to many, especially in countries where PrEP is most needed.
GalNAc-siRNA: The Molecular “Address Label”
siRNA (small interfering RNA) is a tiny molecule that helps turn off specific genes in a cell. If you think of DNA as the master set of genetic instructions in the cell, and mRNA as a copy of one instruction used to make a protein, then the siRNA stops the mRNA from accomplishing its task. It finds the matching mRNA sequence and recruits cellular machinery that destroys it, silencing the gene before a protein can be made.
A useful analogy is that if you think of DNA as a cookbook, then mRNA is a copy of a recipe from that cookbook. siRNA shreds the recipe, so that the meal is never prepared. The cookbook (DNA) remains unchanged, but the siRNA ensures that particular reciped (protein) is never made.
siRNA is tremendously exciting because it can turn off specific genes. The medical challenge has been getting siRNA into the correct cells. RNA is rapidly degraded by enzymes in the bloodstream. It also cannot easily cross cell membranes, can trigger immune responses, and has no built-in mechanism to selectively enter specific cells. Injecting unmodified siRNA into the bloodstream is like mailing a letter with no envelope and no address, and hoping it reaches the correct recipient.
This is where N‑acetylgalactosamine (GalNAc) comes in. GalNAc is a sugar molecule that binds specifically to ASGPR receptors found in high abundance on hepatocytes (liver cells). When siRNA is chemically linked to GalNAc and injected into the bloodstream, these receptors bind the GalNAc and internalize the siRNA into the cell. The address matches, and the siRNA is delivered to the intended destination.
GalNAc-siRNA is an exciting platform for very targeted treatments where hepatocytes (liver cells) are involved. It’s the same delivery vehicle, but the payload is different. This means multiple drugs that treat different diseases can use the same architecture. Two examples:
Fitusiran for hemophilia: In hemophilia, the body is missing or has reduced levels of key clotting proteins (factor VIII in hemophilia A, factor IX in hemophilia B). This results in impaired clot formation and an increased risk of prolonged or spontaneous bleeding.
Rather than replacing the missing clotting factors, fitusiran reduces the production of antithrombin, an anticoagulant produced in the liver, using siRNA-mediated gene silencing. This shifts the balance of the coagulation system toward clot formation.
Because this mechanism acts independently of factor VIII or IX, fitusiran does not rely on replacing those factors. As a result, one drug can be used to treat both hemophilia A and hemophilia B.
In phase 3 trials, fitusiran reduced annualized bleeding rates by approximately 90% compared with traditional treatments, with many patients experiencing no bleeding episodes.
Lepodisiran for elevated Lp(a): Lipoprotein(a), or Lp(a), is a cholesterol-containing particle in the blood with levels that are largely determined by genetics. High Lp(a) levels are associated with an increased risk of cardiovascular disease (e.g., heart attack and stroke). Lp(a) is not meaningfully lowered by standard cholesterol treatments, such as statins, and until recently there have been no targeted therapies to address it.
Lepodisiran is an siRNA therapy designed to reduce the production of Lp(a) at its source in the liver. It targets the mRNA for apolipoprotein(a) in hepatocytes—the key component required to assemble Lp(a). Using a gene-silencing approach, lepodisiran recruits the cell’s RNA-silencing machinery to degrade this mRNA, preventing the protein from being produced. With less apolipoprotein(a) available, the liver generates substantially less Lp(a), lowering circulating levels.
A single injection of lepodisiran reduced Lp(a) levels by about 94%, with effects that persisted for up to a year. A phase 3 cardiovascular outcomes trial is underway.
Conclusion
Each of these examples represents a recognition that the gap between a promising molecule and an effective medicine can be often a delivery problem. Finding a drug that works in a test tube or animal model is challenging enough. Ensuring it survives in the bloodstream, reaches the right cells, acts for long enough to matter, and doesn’t harm you in the process is another challenge entirely. Design and delivery issues have received less attention – but that’s changing. The GalNAc platform alone has dozens of candidates in development targeting other liver-expressed genes. Depot formulations like lenacapavir’s are being explored for other conditions where adherence is a structural barrier to treatment. The molecules are innovative. Increasingly, so are the envelopes we’re delivering them in.