A new investigational medication called HLP003 could change how doctors treat depression by helping patients respond better to their existing antidepressants, rather than waiting until their condition becomes treatment-resistant. HLP003 is a deuterated analogue of psilocin, the active metabolite of psilocybin, and is being developed by Helus Pharma as an add-on therapy for major depressive disorder (MDD).

Why Does This Matter for Depression Treatment?

Depression affects an estimated 23 million people in the United States and more than 300 million people worldwide. The challenge is that at least 50% of those taking a selective serotonin reuptake inhibitor (SSRI) or serotonin-norepinephrine reuptake inhibitor (SNRI), the most common first-line antidepressants, experience an inadequate response to treatment. These patients fall into a frustrating middle ground: their depression isn't fully controlled, but they haven't yet reached the threshold for "treatment-resistant" status, which typically requires failure of two or more medications.

HLP003 targets this larger population of patients who show partial but inadequate response to their current medication. Unlike other psychedelic-derived compounds in development that focus on treatment-resistant depression, which affects only 20% to 25% of patients with MDD, HLP003 is positioned to help the 50% to nearly 60% of patients who need better symptom control earlier in their treatment journey.

"By adding on a complementary medication like HLP003, we are hopefully able to increase their response to their total medication," explained Ken Kramer, PhD, senior vice president and head of medical affairs at Helus Pharma.

Ken Kramer, PhD, Senior Vice President and Head of Medical Affairs at Helus Pharma

How Does HLP003 Work Differently From Current Add-On Treatments?

The medication's design addresses a key limitation of psilocin: it's naturally unstable in the body. HLP003 uses deuteration, a process that stabilizes the psilocin molecule, eliminating the need for the body to convert psilocybin into psilocin before it reaches the brain. This makes the drug more reliable and easier to administer than traditional psilocybin-based approaches.

Currently, doctors have limited options for patients whose antidepressants aren't working well enough. The existing adjunctive treatments carry significant drawbacks that limit their use:

• Lithium: An older augmentation strategy with a narrow therapeutic window and potential kidney and thyroid complications with long-term use.
• Thyroid hormone: Used off-label to enhance antidepressant response, but evidence is mixed and it requires careful monitoring.
• Atypical antipsychotics: These have been used for nearly 30 years as add-on treatments for depression, but they carry a higher risk of weight gain, sedation, akathisia (restlessness), and extrapyramidal symptoms (involuntary movements). They also have a higher discontinuation rate due to side effects compared to placebo.

What Stage Is HLP003 At in Development?

HLP003 is currently being evaluated in a phase 3 clinical trial program as an add-on to an existing SSRI, SNRI, or other antidepressant. Phase 3 trials are the final stage before regulatory review and represent the largest and most rigorous testing phase. Helus Pharma completed enrollment in the phase 3 APPROACH study ahead of schedule in July 2026, a positive sign for the development timeline.

"If we get approved for adjunctive treatment of MDD and we slot into that line of therapy, then we will be able to treat not only a larger portion of patients with MDD, but we see ourselves being used prior to somebody being determined to be treatment resistant," noted Ken Kramer.

Ken Kramer, PhD, Senior Vice President and Head of Medical Affairs at Helus Pharma

What Could This Mean for Patients?

If HLP003 receives FDA approval, it could fundamentally shift how depression is treated. Rather than waiting months or years while trying different medications to reach treatment-resistant status, patients whose current antidepressants provide only partial relief could add HLP003 to their regimen much earlier. This could mean faster symptom improvement and better quality of life for millions of people currently stuck in the gap between inadequate response and treatment-resistant depression.

The fact that HLP003 is being tested as an add-on rather than a replacement for existing antidepressants also means it could work alongside medications patients are already taking, potentially reducing the need to switch treatments entirely. This approach respects the progress patients have already made while addressing remaining symptoms.