MBX Biosciences, Inc.

SecurityMBX / MBX Biosciences, Inc.
InstitutionCitadel Advisors Llc
ManagerKen Griffin
Latest Disclosed Ownership932,833 shares
Latest Disclosed Value $ 27,845,065
Citadel Advisors Llc ownership in MBX / MBX Biosciences, Inc.

On May 15, 2026 - Citadel Advisors Llc filed a 13F-HR form disclosing ownership of 932,833 shares of MBX Biosciences, Inc. (US:MBX) valued at $27,845,065 USD as of March 31, 2026. The entity filed a previous 13F-HR on February 17, 2026 disclosing 809,857 shares of MBX Biosciences, Inc.. This represents a change in shares of 15.18% during the quarter. The current value of the position is $28,246,183 USD.

Citadel Advisors Llc has a history of taking positions in derivatives of the underlying security (MBX) in the form of stock options. The firm currently holds call options representing 9,900 of underlying shares valued at $295,515 USD and put options representing 25,300 of underlying shares valued at $755,205 USD .

Note: prior versions of this page mistakenly stated that the share values represented the number of options, not the number of underlying shares. For more information on the data backing these reports, please see the SEC document Frequently Asked Questions About Form 13F.

Institutional Ownership: 13F and NPORT Filings

The Security and Class in the table below are shown exactly as filed by the investor. We do our best to track continuity of investments through acquisitions, and this will be reflected in the table as changes in names. In addition to descriptive data, performance of the investment is shown over time. To calculate quarterly performance, we first calculate cost basis of the shares purchased during the quarter, then use that to calculate gross profit. Quarterly return is Gross Profit / Starting Portfolio value.

Note that cost basis is calculated and stored in thousands, so small quarterly changes in shares frequently result in a cost basis of zero.

Upgrade to unlock premium data and export to Excel .

Institutional Put/Call Ratios
(Premium Subscription Required)

In addition to reporting standard equity and debt issues, institutions with more than 100MM assets under management must also disclose their put and call option holdings. Since put options generally indicate negative sentiment, and call options indicate positive sentiment, we can get a sense of the overall institutional sentiment by plotting the ratio of puts to calls. The chart to the right plots the historical put/call ratio for this instrument.

Using Put/Call Ratio as an indicator of investor sentiment overcomes one of the key deficiencies of using total institutional ownership, which is that a significant amount of assets under management are invested passively to track indices. Passively-managed funds do not typically buy options, so the put/call ratio indicator more closely tracks the sentiment of actively-managed funds.

MBX / MBX Biosciences, Inc. Historical Put/Call Ratio
Disclosed Equity Positions (from 13F/NPORT Filings)
File
Date
Effective
Date
Form Security Class ID Avg Price
Paid (Est)
Reported
Shares
ΔShares ΔShares
(%)
Reported
Value
(x1000)
ΔValue
(%)
Alloc
(%)
ΔAlloc ΔAlloc (%) Cost Basis
(x1000)
Unrealized
Gain/Loss
(x1000)
2026-05-15 2026-03-31 13F MBX BIOSCIENCES COM 55287L101 932,833 122,976 15.18 27,845 9.02 0.0040
2026-02-17 2025-12-31 13F MBX BIOSCIENCES COM 55287L101 809,857 -186,226 -18.70 25,543 46.53 0.0034
2025-11-14 2025-09-30 13F MBX BIOSCIENCES COM 55287L101 996,083 996,083 17,431 0.0027
2025-05-15 2025-03-31 13F MBX BIOSCIENCES COM 55287L101 0 -220,330 -100.00 0 -100.00
2025-02-14 2024-12-31 13F MBX BIOSCIENCES COM 55287L101 220,330 84,722 62.48 4,061 15.24 0.0007
2024-11-14 2024-09-30 13F MBX BIOSCIENCES COM 55287L101 135,608 135,608 3,523 0.0007
Disclosed Call Options
File
Date
Effective
Date
Form Security Class ID Opt Avg Share
Price
Shares ΔShares
(%)
Value
(x1000)
ΔValue
(%)
Prior Allocation
(%)
Allocation
(%)
ΔAllocation
(%)
Cost Basis
(x1000)
Profit
(x1000)
Return
(%)
2026-05-15 2026-03-31 13F MBX BIOSCIENCES COM Call 9,900 -41.42 296 -44.65 n/a n/a n/a
2026-02-17 2025-12-31 13F MBX BIOSCIENCES COM Call 16,900 -9.14 533 64.00 n/a n/a n/a
2025-11-14 2025-09-30 13F MBX BIOSCIENCES COM Call 18,600 326 n/a n/a n/a
Disclosed Put Options
File
Date
Effective
Date
Form Security Class ID Opt Avg Share Price Shares ΔShares
(%)
Value
(x1000)
ΔValue
(%)
Prior Allocation
(%)
Allocation
(%)
ΔAllocation
(%)
Cost Basis
(x1000)
Profit
(x1000)
Return
(%)
2026-05-15 2026-03-31 13F MBX BIOSCIENCES COM Put 25,300 194.19 755 178.60 n/a n/a n/a
2026-02-17 2025-12-31 13F MBX BIOSCIENCES COM Put 8,600 -92.49 271 -86.47 n/a n/a n/a
2025-11-14 2025-09-30 13F MBX BIOSCIENCES COM Put 114,500 453.14 2,004 748.73 n/a n/a n/a
2025-08-14 2025-06-30 13F MBX BIOSCIENCES COM Put 20,700 236 n/a n/a n/a
Legend
Shares
The total number of shares held by the institution at the end of the reporting period (the effective date). This is provided in the filing.
Value
The total value of the shares as of the effective date. This is provided in the filing.
Avg. Share Price
The weighted average share price of the shares held by the institution. We use FIFO accounting to determine this price
Class
This is the security class as indicated by the filer. There are a variety of values for this field. Common values include "EC" = equity common, "EP" - equity preferred
Allocation (%)
The percent of the institution's portfolio that this position represents. This is provided in NPORT filings. For 13F filings, we calculate it.
Change in Allocation
The change in allocation from the prior reporting period. This is calculated as current allocation - prior allocation.
Change in Allocation (%)
The percent change in allocation from the prior reporting period. This is calculated as (current allocation - prior allocation) / prior allocation.
Cost Basis
The total cost of the shares held. This is calculated as reported shares * avg. share price
Unrealized Gain/Loss
This is the market value of the position as of the effective date minus the cost basis.