OBX Borrow Market Seizes Up as Shorts Pile In

OBX short interest has exploded 885% in a single week. The borrow market is now virtually shut.

Shares short in Obsidian Therapeutics jumped from roughly 149,000 to 1.47 million between September 11 and September 21. Availability has collapsed to just 1.16% — an all-time 52-week low. That means for every 100 shares already borrowed, fewer than 2 remain in the lending pool. The cost to borrow has doubled in a week to 10.44%, the highest level in over a month.

Three Signals, One Direction

The convergence is striking in its consistency. Short interest, cost to borrow, and availability all moved in lockstep over five trading sessions.

Short interest tripled between September 14 and September 17 alone — from 103,000 to 179,000 shares. Then it jumped again to 493,000 on September 18. By September 21 it stood at 1.47 million. Each leg up drew more borrow from the pool, until the pool ran dry.

Cost to borrow followed the same arc. It sat at 5.14% on September 14. By September 21 it had reached 10.44%. Availability, which was still above 107% on September 14 — meaning lenders held more than a full book of shares available relative to shorts — crashed through the floor over the following week.

At 1.16%, availability is now at the tightest point recorded in the past 52 weeks.

What Short Sellers Are Pushing Against

The bear thesis faces real headwinds from the buy side. Leerink Partners initiated coverage on OBX this morning with an Outperform rating and a $32 price target — against a last close of $17.33. That follows two earlier initiations: LifeSci Capital (Outperform, $43 target, August 18) and Piper Sandler (Overweight, $42 target, August 5). William Blair also initiated Outperform coverage on September 9.

Three out of three analyst ratings on record are bullish. The average target across those with stated prices implies substantial upside from current levels.

RA Capital Management holds a 9.7% stake (as last disclosed August 10). Novo Holdings and Deep Track Capital each hold above 5%. All three filed Schedule 13G — passive, not activist — but the shareholder concentration is notable for a small biotech with a tight float.

Data Summary

• Short interest: 1.47M shares (Sept 21), up 885% week-on-week
• Availability: 1.16% — 52-week low; down from 107% just one week prior
• Cost to borrow: 10.44%, up 103% week-on-week
• Stock price: $17.33 (Sept 21), down 4.3% over one week, down 12.9% over one month
• Analyst consensus: Buy (3 initiations, all Outperform/Overweight)
• Next earnings: November 13, 2026
• Key holders: RA Capital 9.7%, Novo Holdings 5.1%, Deep Track Capital 5.4% (as last disclosed)