<PAGE>

                                                                  EXHIBIT 10.5.1

                                 PROMISSORY NOTE

$2,500,000.00                                           Cambridge, Massachusetts
                                                                   June 29, 2006

     FOR VALUE RECEIVED, on the Maturity Date, as such term is defined in
Section 1.1 below, BIOVEX, INC., a Delaware business corporation with its chief
executive office and principal place of business presently at 245 First Street,
Cambridge, Massachusetts 02142 ("BORROWER") promises to pay to the order of
MASSACHUSETTS DEVELOPMENT FINANCE AGENCY, a body politic and corporate created
by Chapter 289 of The Acts of 1998 and established under Massachusetts General
Laws Chapter 23G as amended, ("LENDER") at its principal offices at 160 Federal
Street, Boston, MA 02110, or at such other place as the holder of this note may
from time to time designate in writing, the principal sum of TWO MILLION FIVE
HUNDRED THOUSAND DOLLARS ($2,500,000.00) or such lesser amount advanced by
Lender pursuant to Section 1.1 below (the "LOAN"), or so much thereof then
remaining unpaid, in lawful money of the United States with interest at the rate
or rates set forth below, until fully paid. Borrower further agrees to pay upon
demand made after the occurrence and during the continuance of an Event of
Default, as such term is defined below, all costs, including reasonable
attorneys' fees reasonably incurred in the collection of Borrower's obligations
and the defense, preservation, enforcement or protection of Lender's rights and
remedies under this Note, or in the foreclosure of any mortgage or security
interest now or hereafter securing the same or in any proceedings to otherwise
enforce or protect upon an Event of Default Lender's rights and remedies under
this Note or any security therefor. Interest on this Note shall be computed on
the basis of a year of three hundred sixty (360) days and actual days elapsed.

     1.0. FUNDING; TERM; INTEREST RATE; PAYMENTS.

          1.1. FUNDING; TERM.

          (a) Borrower may request one (1) or more advances of up to the entire
     principal amount of this Note for the installation of the Tenant
     Improvements, as such term is defined in that certain Leasehold Mortgage by
     and between Lender and Borrower, of even date herewith (the "MORTGAGE"), at
     the premises located at 34-A Commerce Way, Woburn, Massachusetts (the
     "PREMISES"). All advances pursuant to this Section 1.1(a) shall be made
     upon satisfaction of those conditions precedent specified in that certain
     Construction Rider to Leasehold Mortgage and Security Agreement by and
     between Borrower and Lender of even date herewith.

          (b) Borrower may request one (1) or more advances, subject to
     availability of funds hereunder as a result of advances made pursuant to
     Section 1.1(a) above and the limitation on the total aggregate advances
     made pursuant to this Section 1.1(b) set forth below, for the purpose of
     purchasing new or used equipment (the "EQUIPMENT") to be operated and
     maintained at the Premises provided that (i) Borrower's request for the
     advance is accompanied by invoices evidencing the purchase of the Equipment
     for which the advance is sought and evidence of delivery of such Equipment
     to the Premises and (ii) such advance does not exceed eighty-five percent
     (85%) of the invoice cost of the Equipment purchased with the funds
     advanced hereunder. Total advances made pursuant to this Section 1.1(b)
     shall not exceed an aggregate of Seven Hundred Thousand Dollars
     ($700,000.00).

          (c) Total advances made pursuant to Section 1.1(a) and (b) for Phase I
     of the Tenant Improvements, as set forth in the plans and specifications
     certified by the architect for the Tenant

<PAGE>

     Improvements to be delivered and approved by Lender (the "PLANS")
     (described as the build-out of an analytical laboratory, a small scale
     Pilot-cGMP manufacturing facility and office space), shall not exceed
     twenty-five percent (25%) of the overall project costs for Phase I. Total
     advances made pursuant to Section 1.1(a) and (b) for Phase I and Phase II
     of the Tenant Improvements as set forth in the Plans shall not exceed
     twenty-five percent (25%) of the overall project costs for the Tenant
     Improvements. Each request for an advance hereunder shall constitute a
     representation and warranty by Borrower that all applicable conditions to
     such advance have been satisfied on the date of such request. Provided no
     Event of Default exists on the date of each request for an advance and no
     event or circumstance exists on such date which with the passage time, or
     notice, or both would result in an Event of Default, Lender shall advance
     the full requested amount within ten (10) business days of the date of such
     request (the "FUND DATE"). The term of this Note shall commence on July 1,
     2006 (the "TERM COMMENCEMENT DATE") and shall mature on June 1, 2013 (the
     "MATURITY DATE")

          1.2. PAYMENTS OF PRINCIPAL AND INTEREST. Payments of interest only
     during the first eighteen (18) months of the term of this Note, at the rate
     provided in Section 1.3 below, shall be due and payable and shall commence
     on the first day of the calendar month next following the first Fund Date
     (the "PAYMENT START DATE"). Each such payment of accrued interest shall be
     paid in arrears. Thereafter, subject to the adjustment in payment provided
     in Section 1.4 below, payments of principal and interest at the rate
     provided in Section 1.3 below shall commence on the nineteen (19) month
     anniversary of the Term Commencement Date (the "AMORTIZATION COMMENCEMENT
     DATE") and shall continue on the same date of each successive month
     thereafter through and including the Maturity Date, at which time all
     remaining principal and accrued interest shall be paid. Each such monthly
     payment shall include principal equal to the aggregate then outstanding
     principal amount divided by the number of months then remaining in the term
     plus accrued interest on the outstanding principal balance at the rate
     provided in Section 1.3 (or, if applicable, Section 1.4) and shall be paid
     in arrears.

          1.3. INTEREST RATE. So long as no Event of Default (hereafter defined)
     has occurred and is continuing (but subject to applicable cure or grace
     periods), and subject to the adjustment in the interest rate provided in
     Section 1.4 below, the principal outstanding hereunder from time to time
     shall bear interest, beginning on the Payment Start Date, on the
     outstanding principal amount of this Note at a rate of six percent (6%) per
     annum (based upon a three hundred and sixty (360) day year and actual days
     elapsed).

          1.4. ADDITIONAL INTEREST AMOUNT, PAYMENT AND ADJUSTMENT TO INTEREST
     RATE.

          (a) Upon the occurrence of any Trigger Event (defined in Section
     1.4(c) below), an Additional Interest Amount, as defined below, shall be
     calculated in respect of the outstanding principal amount of this Note for
     a period (the "ADDITIONAL INTEREST COMPUTATION PERIOD") commencing on the
     Payment Start Date and ending on the Additional Interest Payment Date
     (defined below). The "ADDITIONAL INTEREST AMOUNT" shall be equal to the
     difference between (i) the aggregate interest amount that would have
     accrued on the outstanding principal amount of this Note during the
     Additional Interest Computation Period at an annual interest rate equal to
     ten percent (10%) based upon a three hundred and sixty (360) day year and
     actual days elapsed, and (ii) the aggregate amount of interest due and
     payable to Lender hereunder during the same period pursuant to Section 1.3
     of this Note. The Additional Interest Amount shall be paid to Lender in one
     (1) lump sum on the due date (the "ADDITIONAL INTEREST PAYMENT DATE") which
     shall be the date of the regularly scheduled monthly installment next
     following the date of the Trigger Event,


                                       2

<PAGE>

     which Additional Interest Amount shall be paid together with such regularly
     scheduled monthly installment.

          (b) Beginning with the Additional Interest Payment Date and continuing
     thereafter for the remainder of the term of this Note, interest on any
     principal outstanding hereunder shall accrue and be payable in arrears
     hereunder at ten percent (10%) per annum together with the principal sum
     due pursuant to Section 1.2.

          (c) For purposes of this Note, a "TRIGGER EVENT" shall mean any of the
     following: (i) the date that Borrower's audited financial statements
     reflect achievement by Borrower of EBITDA (calculated in the manner set
     forth) greater than or equal to ten thousand dollars ($10,000) for any
     complete twelve-month fiscal year, (ii) the Borrower is acquired or merged,
     whether Borrower is the surviving entity or not, during the term of this
     Note, or (iii) the Borrower completes an initial public offering of its
     stock. For purposes of this Note, "EBITDA" shall mean for any complete
     twelve-month fiscal year the sum of (i) net income (or loss) of Borrower
     for such period (excluding extraordinary gains and non-cash gains,
     including without limitation any one-time or lump sum license fees and
     development, milestone or similar payments made by any third party as part
     of a strategic partnership or similar arrangement), plus (ii) all interest
     expense of Borrower for such fiscal year, plus (iii) all charges against
     income of Borrower for such fiscal year for federal, state and local taxes,
     plus (iv) depreciation expenses for such fiscal year, plus (v) amortization
     expenses for such period, all determined in accordance with generally
     accepted accounting principles in the United States of America in effect
     from time to time applied on a consistent basis and with reference to the
     Borrower's audited financial statements with respect to such fiscal year.

     2.0. DEFAULT RATE. To the extent allowed by applicable law, after the
occurrence of any Event of Default and during the continuation thereof (and
after giving effect to any applicable grace or cure periods), after the Maturity
Date, or after judgment has been rendered on this Note, all outstanding
principal and unpaid interest shall bear, until paid, interest at a rate per
annum equal to five (5%) percentage points greater than that which would
otherwise be applicable assessed retroactive to the date that the Event of
Default first occurs (the "DEFAULT RATE").

     3.0. LATE CHARGE. If a regularly scheduled payment is ten (10) days or more
late, Borrower will be charged five percent (5%) of the unpaid portion of the
regularly scheduled payment or ten dollars ($10.00), whichever is greater. If
Lender demands payment of this Loan after the occurrence and during the
continuation of an Event of Default (after giving effect to any applicable grace
or cure periods), and Borrower does not pay the Loan within fifteen (15) days
after Lender's demand, Borrower will be charged either five percent (5%) of the
unpaid principal amount plus accrued unpaid interest or ten dollars ($10.00),
whichever is greater.

     4.0. EXPENSES. Borrower further promises to pay to Lender, as incurred, and
as an additional part of the unpaid principal amount, upon demand made after the
occurrence and during the continuance of an Event of Default all costs, expenses
and reasonable attorneys' fees reasonably incurred by Lender: (a) in the
protection, modification, collection, defense or enforcement of all or part of
this Note or any guaranty hereof; or (b) in the foreclosure or enforcement of
any mortgage or security interest which may now or hereafter secure the debt
hereunder, or (c) with respect to any action taken to protect, defend, modify or
sustain the lien of any such mortgage or security agreement; or (d) with respect
to any litigation or controversy arising from or connected with this Note or any
mortgage or security agreement or collateral which may now or hereafter secure
this Note; or (e) with respect to any act to protect defend, modify, enforce or
release any of its rights or remedies with regard to, or otherwise effect
collection of,


                                       3

<PAGE>

any collateral which may now or in the future secure this Note or with regard to
or against Borrower or any endorser, guarantor or surety of this Note.

     5.0 OPTIONAL PREPAYMENT; TERMINATION. (a) Borrower may at any time elect to
prepay the unpaid amount of this Note, or any part thereof, without penalty or
premium; provided, however, that, notwithstanding anything hereinto the
contrary, in the event that any Additional Interest Amount is due and payable
pursuant to Section 1.4 hereof, any such prepayment shall first be applied to
(i) the Additional Interest Amount then owed, then to (ii) any unpaid Expenses
required by Section 4.0 above, then to (iii) Late Fees, if any are due, then to
(iv) Default Rate interest, if any is due, then to (v) regularly accrued but
unpaid interest pursuant to Section 1.3 above, and then finally to (vi) the
principal amount outstanding.

     (b) Borrower may terminate the Loan facility at any time provided that (i)
no principal, interest, fees, expenses or other amounts remain outstanding
hereunder and (ii) Borrower provides Lender with written notice of its election
to terminate this Loan.

     6.0. DEFAULT. The happening of any of the following events or conditions
shall constitute an "EVENT OF DEFAULT" under this Note:

          6.1. Failure to make any payment of principal or interest on any sum
     due under this Note within five (5) days after written notice that the same
     is due and payable; provided, however, that Lender shall not be required to
     provide such written notice more than twice in any twelve (12) month
     period.

          6.2. Failure by Borrower to observe or perform any covenant contained
     herein or a default or the occurrence of an event of default in any
     agreement between Borrower and Lender in connection herewith, including
     that certain Leasehold Mortgage and Security Agreement by and between
     Borrower and Lender of even date herewith (the "MORTGAGE"), beyond the
     applicable grace or cure period (or, if no such grace or cure period is
     specified, then beyond thirty (30) days following the occurrence of any
     such default or event of default), provided, however, that if such default
     or event of default cannot be cured within the applicable grace or cure
     period, provided Borrower commences cure within thirty (30) days of such
     default or event of default and continuously and diligently pursues such
     cure thereafter, such applicable grace or cure period shall be extended an
     additional sixty (60) days.

          6.3. A material default or the occurrence of a material event of
     default under that certain Commercial Lease by and between Borrower and
     Cummings Properties, LLC ("LANDLORD") dated December 2, 2005, including all
     addenda and riders thereto (the "LEASE") beyond the applicable grace or
     cure period. A "material" default as provided in this Section 6.3 mean any
     breach and/or default of any covenant, term or condition of the Lease which
     in the Lender's good faith reasonable belief evidences an unacceptable
     decline in the Borrower's ability to fully perform its obligations under
     this Note.

          6.4 Failure of Borrower to maintain the term of the Lease throughout
     the full term of this Note except for any termination arising from casualty
     or condemnation.

          6.5. Any cancellation, termination, surrender or transfer of the
     Lease, or any amendment or modification thereof without the Lender's prior
     written consent.


                                       4

<PAGE>

          6.6. Any representation or warranty made by Borrower herein or in any
     agreement executed in connection herewith, including the Mortgage, or any
     statement, certificate or other data furnished by Borrower in connection
     herewith or with such agreements, proves at any time to be incorrect in any
     material respect.

          6.7. A final judgment or judgments for the payment of money shall be
     rendered against Borrower in an amount, individually or in the aggregate,
     of at least two hundred fifty thousand dollars ($250,000.00), and any such
     judgment shall remain unsatisfied and in effect for any period of thirty
     (30) consecutive days without a stay of execution, and the Lender holds the
     good faith belief that such unsatisfied judgment or unstayed execution is
     materially adverse to the condition (financial or otherwise) of the
     Borrower or the Lender's collateral for this Note.

          6.8. Borrower shall: (a) apply for or consent to the appointment of a
     receiver, trustee or liquidator of all or a substantial part of any of its
     assets; (b) admit in writing its inability to pay its debts as they mature;
     or (c) file or permit the filing of any petition, case arrangement,
     reorganization, or the like under any insolvency or bankruptcy law, or the
     adjudication of it as a bankrupt, or the making of an assignment for the
     benefit of creditors or the consenting to any form or arrangement for the
     satisfaction, settlement or delay of debt or the appointment of a receiver
     for all or any part of its properties.

          6.9. An order, judgment or decree shall be entered, or a case shall be
     commenced, against Borrower, without its application, approval or consent
     by any court of competent jurisdiction, approving a petition or permitting
     the commencement of a case seeking reorganization or liquidation of
     Borrower or appointing a receiver, trustee or liquidator of Borrower, or of
     all or a substantial part of the assets of Borrower, and Borrower, by any
     act, indicates its approval thereof, consent thereto, or acquiescence
     therein, or such order, judgment, decree or case shall continue unstayed
     and in effect for any period of sixty (60) consecutive days, or an order
     for relief in connection therewith shall be entered.

          6.10. If Borrower shall dissolve or liquidate, or be dissolved or
     liquidated, or cease to legally exist or failure of the Borrower to pay the
     Additional Interest Amount due from any merger or consolidation
     constituting a Trigger Event.

          6.11. Failure of Borrower to pay any other indebtedness or obligation,
     or if any such other indebtedness or obligation shall be accelerated, or if
     there exists any event of default under any instrument, document or
     agreement governing, evidencing or securing such other indebtedness or
     obligation, in any event, in an amount, individually or in the aggregate,
     of at least two hundred fifty thousand dollars ($250,000.00); and any such
     amount remains unpaid or any such indebtedness or obligations remains
     accelerated or any such event of default remains uncured, in each case, for
     any period of thirty (30) consecutive days; and the Lender holds the good
     faith belief that such unpaid indebtedness or event of default is
     materially adverse to the condition (financial or otherwise) of the
     Borrower or the Lender's collateral for this Note.

          6.12. Intentionally deleted.

          6.13. The Borrower, for any reason, fails to maintain, or ceases the
     operations necessary for the development, application and manufacturing of
     OncoVEX, ImmunoVEX and other cancer and theraputic vaccines on the
     Premises.


                                       5

<PAGE>

          6.14. Failure of the Borrower to pay the Additional Interest Amount
     due from any sale by Borrower of all or substantially all its assets, or a
     change in the ownership of the capital stock of the Borrower, constituting
     a Trigger Event.

     Upon and after an Event of Default, the whole of said indebtedness, both
principal and interest, and including any other sums which may become due under
this Note, shall, at the option of the holder of this Note, immediately become
due and payable without presentment, demand, protest, notice of protest, or
other notice of dishonor of any kind, all of which are hereby expressly waived
by Borrower.

     7.0. MAXIMUM PERMISSIBLE INTEREST RATE. Borrower shall not be obligated to
pay and Lender shall not collect interest at a rate higher than the maximum
permitted by law or the maximum that will not subject Lender to any civil or
criminal penalties. If, because of the acceleration of maturity the payment of
interest in advance or any other reason, Borrower is required, under the
provisions hereof, pursuant to the provisions of any other agreements,
instruments, documents, security agreements, mortgages, financing statements,
and supplements thereto and relating to the Loan, or entered into between
Borrower in favor of, or with, Lender, at any time, for any purpose (the "LOAN
DOCUMENTS") or otherwise, to pay interest at a rate in excess of such maximum
rate, the rate of interest under such provisions shall immediately and
automatically be reduced to such maximum rate and any payment made in excess of
such maximum rate shall be applied to principal outstanding hereunder or, if
received by applicable law, shall be returned to Borrower.

     8.0. SOURCE OF LOAN; LIMITED RECOURSE. Borrower hereby acknowledges that
the Loan is being made by Lender from the Commonwealth of Massachusetts's
Emerging Technology Fund created pursuant to Section 27 of Chapter 23G of
Massachusetts General Laws (the "FUND") which is administered by Lender. In
consideration of the Lender's agreement to make the Loan, to the extent that the
Borrower ever has any off-sets, defenses or claims against the Lender, its
subsidiaries, affiliates, any members of the Fund's advisory committee, parents,
officers, directors, employees, agents, predecessors, successors and assigns,
both present and former (collectively, the "LENDER AFFILIATES"), the Borrower
and its partners, subsidiaries, affiliates, parents, officers, directors,
employees, agents, heirs, successors, assigns, and executors, (collectively, the
"OBLIGOR PARTIES"), agree that any recourse an Obligor Party may have against
the Lender or any Lender Affiliate will be limited to the Fund for any action
and actions, cause and causes of action, suits, debts, controversies, damages,
judgments, executions, claims and demands whatsoever asserted or unasserted, in
contract, tort, law or in equity which the Obligor Parties may have upon or
against the Lender or any Lender Affiliate by reason of any matter, cause,
causes or thing whatsoever including, without limitation, to any claim that
relates to, in whole or in part, directly or indirectly (a) the making or
administration of the Loan, including, without limitation, such claims and
defenses based on mistake, usury, misrepresentation, or negligence; (b) any
covenants, agreements, duties, or obligations set forth in the Loan Documents;
(c) the actions or omissions of the Lender or any Lender Affiliate in connection
with the initiation or continuing exercise of any right or remedy contained in
the Loan Documents or at law or in equity; (d) lost profits; (e) loss of
business opportunity; (f) increased financing costs; (g) increased legal or
administrative fees; or (h) damages to business reputation, but excluding the
gross negligence and willful misconduct of the Lender.

     9.0. REPLACEMENT DOCUMENTS. Upon receipt of an affidavit of an officer of
Lender as to the loss, theft, destruction or mutilation of the Note or any other
security document(s), together with an agreement reasonably satisfactory to the
Borrower to indemnify the Borrower from any loss incurred by it in connection
therewith, and, in the case of any such loss, theft, destruction or mutilation,
upon surrender and cancellation of such Note or other document(s), Borrower will
issue, in lieu thereof, a replacement Note or other document(s) in the same
principal amount thereof and otherwise of like tenor.

     10.0 INTENTIONALLY DELETED.


                                       6

<PAGE>

     11.0. CONSENT TO JURISDICTION. Borrower hereby agrees that any state or
local court of the Commonwealth of Massachusetts or any United States District
Court for the District of Massachusetts or, at the option of Lender, any court
in which Lender shall initiate legal or equitable proceedings and which has
subject matter jurisdiction over the matter in controversy, shall have exclusive
jurisdiction to hear and determine any claims or disputes between Borrower and
Lender pertaining directly or indirectly to this Note or to any matter arising
in connection with this Note.

     12.0. WAIVERS. Borrower agrees that no delay or failure on the part of the
holder in exercising any power, privilege, remedy, option or right hereunder
shall operate as a waiver thereof or of any other power, privilege, remedy or
right; nor shall any single or partial exercise of any power, privilege, remedy,
option or right hereunder preclude any other or future exercise thereof or the
exercise of any other power, privilege, remedy, option or right. The rights and
remedies expressed herein are cumulative, and may be enforced successively,
alternately, or concurrently and are not exclusive of any rights or remedies
which holder may or would otherwise have under the provisions of all applicable
laws, and under the provisions of all agreements between Borrower and Lender.

     Borrower hereby waives presentment, demand, notice, protest and all other
demands and notices in connection with the delivery, acceptance, performance,
default or enforcement of this Note. Borrower hereby assents to any extension or
postponement of the time of payment or any other indulgence, to the addition or
release of any party or person primarily or secondarily liable, and to the
addition, release and/or substitution of all or any portion of any collateral
now or hereafter securing this Note.

     BORROWER AND LENDER MUTUALLY HEREBY KNOWINGLY, VOLUNTARILY AND
INTENTIONALLY WAIVE THE RIGHT TO A TRIAL BY JURY IN RESPECT OF ANY CLAIM BASED
HEREON, ARISING OUT OF, UNDER OR IN CONNECTION WITH THIS NOTE OR ANY OTHER
DOCUMENTS CONTEMPLATED TO BE EXECUTED IN CONNECTION HEREWITH OR ANY COURSE OF
CONDUCT, COURSE OF DEALING, STATEMENTS (WHETHER ORAL OR WRITTEN) OR ACTIONS OF
ANY PARTY. THIS WAIVER CONSTITUTES A MATERIAL INDUCEMENT FOR LENDER TO MAKE THE
LOAN AND ACCEPT THIS NOTE.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]


                                       7

<PAGE>

     This Note is executed as a sealed instrument and shall be governed by and
construed in accordance with the laws of the Commonwealth of Massachusetts.

WITNESS:                                BORROWER:

                                        BIOVEX, INC.


/s/ [signature illegible]               By: /s/ Philip Astley-Sparke
-------------------------------------       ------------------------------------
                                            Philip Astley-Sparke,
                                            President
                                            Duly Authorized


                                       8